There was a previous attempt to do this in D39215. Back then the LLDB_TEST_CXX_COMPILER was apparently still in use. Today it's unused and we could use the chance and finish the work.
Following up from D56400:
- add LLDB_TEST_COMPILER setting, while still supporting LLDB_TEST_C/CXX_COMPILER
- remove LLDB_TEST_USE_CUSTOM_C/CXX_COMPILER: overriding LLDB_TEST_COMPILER alone is sufficient
- use LLDB_TEST_COMPILER_IS_DEFAULT to determine whether or not to replace configuration names
- update documentation to reflect changes
As we import targets like clang and dsymutil anyway, we might use generator expressions to fetch paths of the required tools.
The problem is not that generator expressions don't work in the multi-config case. We just need a generation-time step for each lit.site.cfg (or at least those that use LLDB_DOTEST_ARGS). That may like this: