Fix cache collision with object file and precompiled headers

[jimis]

When compiling with PCH enabled, it happens that some times object files end up with PCH content, or that .gch files end up with object code.

Simplest way to reproduce the problem:

$ touch empty.c
$ gcc -c -o out1 empty.c
$ gcc -x c-header -c -o out2 empty.c
$ file out1 out2
out1: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
out2: GCC precompiled header (version 014) for C

The two files are different. But if we feed these compilations to sccache, they lead to the same result:

$ sccache gcc -c -o out3 empty.c
$ sccache gcc -x c-header -c -o out4 empty.c
$ file out3 out4
out3: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
out4: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

Same thing is reproducible with c++ compiler and -x c++-header argument.

The reason is that the hash string that identifies each command line is the same.

With this patch, compilation of C files is always differentiated from compilations of same C-Header files. And compilation of C++ files is always differentiated from compilations of same C++-Header files.

Fixes #1851.

[jimis]

This is ready for review.

The weirdest thing happened while I was running the integration tests on my fork: The "clang" integration test failed (the last --stats invocation showed 0 cache hits). Then I click "rerun failed tests" without changing anything and it passed (it then showed 2 cache hits).
Are the integration tests known to be flaky?

[sylvestre]

Are the integration tests known to be flaky?

yes, the cache might be full at time

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 40.61%. Comparing base (0cc0c62) to head (b881869).
Report is 80 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/c.rs	33.33%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2268      +/-   ##
==========================================
+ Coverage   30.91%   40.61%   +9.70%     
==========================================
  Files          53       54       +1     
  Lines       20112    20746     +634     
  Branches     9755     9641     -114     
==========================================
+ Hits         6217     8427    +2210     
- Misses       7922     8150     +228     
+ Partials     5973     4169    -1804     

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[sylvestre]

nice easy fix, well done :)