LLVM_USE_CRT_RELEASE/DEBUG seems to have been broken with the update to require CMake 3.20 #62739
Description
LLVM has the CMake flag LLVM_USE_CRT_RELEASE (and a debug equivalent) that can add e.g. the /MT compiler flag to select the static C runtime. With the update to require a newer version of CMake, this seems to have stopped working.
I'm using Cmake 3.21 with clang-cl and using -DLLVM_USE_CRT_RELEASE=MT, and found that after the update, clang was getting passed the /MT flag but then after that the -MD flag was being added by CMake. This seems to be because of https://cmake.org/cmake/help/latest/policy/CMP0091.html, a new feature that does a similar runtime selection at the CMake level. I found that adding -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded made it start working again.
So, probably LLVM_USE_CRT_{RELEASE,DEBUG} needs to be updated somehow to work with the new policy, or perhaps it can just be removed in favor of the CMake version.