[CMake] Don't override required compiler flags in the runtimes build

Ensure that HandleLLVMOptions adds all necessary required flags,
including -Wno-error when building with LLVM_ENABLE_WERROR enabled.

Differential Revision: https://reviews.llvm.org/D58092

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353790 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed