[libc++] Always build with -fvisibility=hidden

Summary:
This avoids symbols being accidentally exported from the dylib when they
shouldn't. The next step is to use a pragma to apply hidden visibility
to all declarations (unless otherwise specified), which will allow us
to drop the per-declaration hidden visibility attributes we currently
have.

This also has the nice side effect of making sure the dylib exports the
same symbols regardless of the optimization level.

PR38138

Reviewers: EricWF, mclow.lists

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D62868

llvm-svn: 368703