[CMake] Add -stdlib=libc++ to host Clang build flags before performing any header search

If LLVM_ENABLE_LIBCXX is specified, we should append -stdlib=libc++ to build
flags as early as possible, in particular, before we check for header presence
(as -stdlib=libc++ modifies header lookup rules). Otherwise we can find a header
at configure time (w/o -stdlib=libc++) but fail to find it at build time
(with -stdlib=libc++). See PR18569 for more details.

llvm-svn: 200744