[CMake] Introduce llvm_update_compile_flags(target_name) to update compile flags in target properties.

FIXME: Just add_unittest() is using it.
FIXME: Cooperate with source properties.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198683 91177308-0d34-0410-b5e6-96231b3b80d8