CMake configure compiler-rt got broken as a result of following commit:
d3925e65a7
This patch fixes the break by porting the above commit for clang-cl.
This problem was not caught on Windows buildbots beacuase it appeared
when compiler-rt was included via LLVM_ENABLE_PROJECTS while buildbots
include compiler-rt project using LLVM_ENABLE_RUNTIMES flag.