[clang] [CMake]Fully delete the deprecated LLVM_USE_CRT* (PR #66850)

[Mike Hommey via cfe-commits]

glandium wrote:

This unveiled an interesting problem in our builds: `llvm-config --cxxflags` doesn't include the `/MT`/`/MD` flag, so when building something that links with one of the .lib from LLVM, you can end up with a failure because you're not building with the same flag as LLVM was built with. Which used to be MT and with this change is now MD.

https://github.com/llvm/llvm-project/pull/66850