[Clang][test] Add .i files for test discovery

[saudi]

Summary

The .i files in the clang tests (2 files) were not run by lit :

clang/test/CodeGen/debug-info-preprocessed-file.i
clang/test/FrontEnd/processed-input.i

[hans]

Looks good to me.

Do you have commit access, or would you like me to commit this for you?

[aganea]

I've asked Chris commit access for Sylvain. I'd like him to be able to commit directly, we have quite a few other things coming along :)
Sylvain is working on issues related to debugging cross-compiled executables on Linux (when compiled on Windows) and also debugging builds made from OBJs compiled on other machines. This is related to Nico's post about deterministic builds, however things don't work the same with Fastbuild (which is P2P) and there are a few issues we'd like to address.

clang/test/lit.cfg.py

 
 # testFormat: The test format to use to interpret tests.
 #
 # For now we require '&&' between commands, until they get globally killed and
 # the test runner updated.
 config.test_format = lit.formats.ShTest(not llvm_config.use_lit_shell)
 
 # suffixes: A list of file extensions to treat as test files.
-config.suffixes = ['.c', '.cpp', '.cppm', '.m', '.mm', '.cu',
+config.suffixes = ['.c', '.cpp', '.i', '.cppm', '.m', '.mm', '.cu',
                    '.ll', '.cl', '.s', '.S', '.modulemap', '.test', '.rs', '.ifs']
 
 # excludes: A list of directories to exclude from the testsuite. The 'Inputs'
 # subdirectories contain auxiliary inputs for various tests in their parent
 # directories.
 config.excludes = ['Inputs', 'CMakeLists.txt', 'README.txt', 'LICENSE.txt', 'debuginfo-tests']
 
 # test_source_root: The root path where tests are located.