[libc++] Localize common build flags into a single CMake function

Also, set those flags for the cxx_experimental target. Otherwise,
cxx_experimental doesn't build properly when neither the static nor
the shared library is compiled (yes, that is a weird setup).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373808 91177308-0d34-0410-b5e6-96231b3b80d8