[CMake] Don't use llvm_replace_compiler_option

Building compiler-rt standalone outside of the main llvm
tree is supported, and in this case, the llvm cmake modules
are unavailable. (And even if they are available, it requires
including LLVMProcessSources.cmake, which currently isn't
included.)

This fixes building compiler-rt standalone with MSVC/clang-cl.

Differential Revision: https://reviews.llvm.org/D58767

llvm-svn: 355237