[libc++] Make the modules-related flags PUBLIC instead of PRIVATE

It appears that those need to be propagated to targets that use libc++
as well, otherwise they don't build properly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373773 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed