[libc] Warn on use of global constructors in `libc`

Clang supports the `-Wglobal-constructors` flag which will indicate if a
global constructor is being used. The current goal in `libc` is to make
the constructors `constexpr` to prevent this from happening with
straight construction. However, there are many other cases where we can
emit a constructor that this won't catch. This should give warning if
someone accidentally introduces a global constructor.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D155721