Allows using different compilers with different flags for different languages.
For example Clang with GNU-like commandline for CXX and MSVC as host compiler
for CUDA.
Should help with #21914.
Migrate from the old `<os>-<cc>.cmake` layout to the modern
`<os>-<id>-<lang>.cmake` layout. Keep settings common to C and C++ in a
`Windows-OpenWatcom.cmake` helper module with an include blocker.
For now just add both C and CXX settings in the helper module.