[CMake] LLDB_TEST_(C/CXX)_COMPILER in patches for standalone and testing

Hi all

I put a number of patches for review that are related to LLDB standalone builds and testing. They mostly clean up things that confused me while investigating the mysterious problem with the cxx target dependency [1][2].

One of the changes [4] has relevant effects on CMake parameters: it deprecates LLDB_TEST_C/CXX_COMPILER in favor of a unified LLDB_TEST_COMPILER (for details on the change itself, let’s discuss in the review).
As this may affect build bot configurations etc., the old parameters are only marked deprecated for now and will continue to work. I imagined to phase them out until the 8.0 release. What do you think?

The other related reviews are linked below.