[lldb][test] Pick correct frontend depending on test language (#212848)

Clang configuration files may contain frontend-specific options.
So building Objective C files with the clang++ may fail due to an option not being available for the C frontend.
Clean up the code slightly and pick the correct compiler based on the file extension.

GitOrigin-RevId: 8f94ddb3e730a613885ef36384ebc50c9146ed85