CodeChecker cannot execute if there isn't a clang binary on the system

[whisperity]

Describe the bug
If clang is not found in PATH anywhere at all, the analyze and check commands fail to go because they try to always read a Clang version.

CodeChecker version

(Note that even running something as simple and completely orthogonal as a package version check wants to access Clang!!!)

╰─ ~/CodeChecker/build/CodeChecker/bin/CodeChecker version
CodeChecker analyzer version:
[WARNING 2021-01-13 17:56] - 'clang' binary can not be found in your PATH!
---------------------------------------------------------------
Kind                 | Version
---------------------------------------------------------------
Base package version | 6.16.0
Package build date   | 2021-01-13T17:50
Git commit ID (hash) | 9d23b7ba6d4c149a6fabf21579f08fc60ef4df1d
Git tag information  | 6.16
---------------------------------------------------------------

CodeChecker web version:
------------------------------------------------------------------------
Kind                          | Version
------------------------------------------------------------------------
Base package version          | 6.16.0
Package build date            | 2021-01-13T17:50
Git commit ID (hash)          | 9d23b7ba6d4c149a6fabf21579f08fc60ef4df1d
Git tag information           | 6.16
Server supported API (Thrift) | 6.37
Client API (Thrift)           | 6.37
------------------------------------------------------------------------

To Reproduce
Steps to reproduce the behaviour:

1. Download and build CodeChecker.
2. Install clang-tidy from the package manager, or have a binary release of Clang-Tidy, or compile it locally. Either way, do not have clang in the PATH, only clang-tidy.
3. Try to run an check --analyzers clang-tidy ... or analyze --analyzers clang-tidy.
4. Observe error:

╰─ which clang
clang not found

╰─ which clang-tidy
~/LLVM/Build/bin/clang-tidy

╰─ ~/CodeChecker/build/CodeChecker/bin/CodeChecker check -l compile_commands.json --analyzers clang-tidy
'clang' binary can not be found in your PATH!
'clang' binary can not be found in your PATH!
[WARNING 2021-01-13 17:56] - 'clang' binary can not be found in your PATH!
[ERROR 2021-01-13 17:56] - Running check failed.
Traceback (most recent call last):
  File "~/CodeChecker/build/CodeChecker/cc_bin/../lib/python3/codechecker_analyzer/cmd/check.py", line 819, in main
    analysis_exit_status = analyze_module.main(analyze_args)
  File "~/CodeChecker/build/CodeChecker/lib/python3/codechecker_analyzer/cmd/analyze.py", line 937, in main
    analyzer_env)
  File "~/CodeChecker/build/CodeChecker/lib/python3/codechecker_analyzer/analyzers/clangsa/version.py", line 73, in get
    errors="ignore")
  File "/usr/lib/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.6/subprocess.py", line 729, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1278, in _execute_child
    executable = os.fsencode(executable)
  File "/usr/lib/python3.6/os.py", line 800, in fsencode
    filename = fspath(filename)  # Does type-checking of `filename`.
TypeError: expected str, bytes or os.PathLike object, not NoneType
Traceback (most recent call last):
  File "~/CodeChecker/build/CodeChecker/cc_bin/../lib/python3/codechecker_analyzer/cmd/check.py", line 819, in main
    analysis_exit_status = analyze_module.main(analyze_args)
  File "~/CodeChecker/build/CodeChecker/lib/python3/codechecker_analyzer/cmd/analyze.py", line 937, in main
    analyzer_env)
  File "~/CodeChecker/build/CodeChecker/lib/python3/codechecker_analyzer/analyzers/clangsa/version.py", line 73, in get
    errors="ignore")
  File "/usr/lib/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.6/subprocess.py", line 729, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1278, in _execute_child
    executable = os.fsencode(executable)
  File "/usr/lib/python3.6/os.py", line 800, in fsencode
    filename = fspath(filename)  # Does type-checking of `filename`.
TypeError: expected str, bytes or os.PathLike object, not NoneType
Traceback (most recent call last):
  File "~/CodeChecker/build/CodeChecker/cc_bin/CodeChecker.py", line 141, in main
    sys.exit(args.func(args))
  File "~/CodeChecker/build/CodeChecker/cc_bin/../lib/python3/codechecker_analyzer/cmd/check.py", line 850, in main
    return analysis_exit_status
UnboundLocalError: local variable 'analysis_exit_status' referenced before assignment

Expected behaviour
CodeChecker acknowledges the fact that Clang does not exist on the system and goes on with only running Clang-Tidy.

Desktop (please complete the following information)

• OS: Ubuntu 18.04.5 LTS

Additional context
This issue, together with #2670 makes it really hard to use CodeChecker for actively testing Tidy (and Tidy-only) contributions. I do not wish to have Clang SA built on the system because it's a lot of effort to always rebuild features that are not being used.

The culprit in the source code is here:

codechecker/analyzer/codechecker_analyzer/cmd/analyze.py

Lines 933 to 937 in 9d23b7b

	analyzer_clang_binary = \
	context.analyzer_binaries.get(
	clangsa.analyzer.ClangSA.ANALYZER_NAME)
	analyzer_clang_version = clangsa.version.get(analyzer_clang_binary,
	analyzer_env)

[whisperity]

Note that even removing the "name" for the clangsa analyzer from the package_layout.json file, i.e. deleting this line:

codechecker/config/package_layout.json

Line 4 in 9d23b7b

"clangsa": "clang",

does not solve the issue. The 'clang' binary can not be found in your PATH! prints (both the non-WARING and the [WARNING] ones) go away, but the three exception traces (and the fact that no analysis is run) remains the same.