[analyzer] Handle relative file paths in compilation database

[csordasmarton]

Let's suppose that we have the following directory stucture:

|- .codechecker
  |- compilation_commands.json
  |- reports
|- main.cpp

And all the file paths in the compilation database are relative:

[
  {
    "file": "main.cpp",
    "command": "g++ -c main.cpp -o /dev/null",
    "directory": "."
  }
]

If we run the CodeChecker analyze command on this project, ClangSA analyzer
will generate a plist file where the file path will be relative to the directory
where the analyzer was called on.

After the plist files are produced, report converter will try to post-process these
files and creates absolute paths from the relative paths.

With this patch now we will create absolute paths based on the following values:

• Current directory (os.getcwd()) where the CodeChecker command was executed.
• The value of the directory key from the build action (in the example above it is .).
• File path in the plist file.

If any of the above mentioned values are already absolute paths during the join it will
skip the previous values from the file path.

[bruntib]

This patch looks good.
In the meantime I was thinking how is it possible that a compilation_database.json contains relative directory paths. Probably it is possible with intercept-build or some other logger. This patch technically makes analysis possible with such a build log as input. However, it's important to know for the users that in this case analyzer should be executed in the same directory as the logger was run (i.e. that paths are relative to). This could be mentioned in some documentation.

Maybe here we could mention that CodeChecker supports analysis based on log files of other loggers, but in case of relative paths we should make sure that analysis is executed from the correct directory: https://github.com/Ericsson/codechecker/blob/master/docs/analyzer/user_guide.md#analyze-