[PGO] Make emitted symbols hidden

This was reverted because it was breaking when targeting Darwin which
tried to export these symbols which are now hidden. It should be safe
to just stop attempting to export these symbols in the clang driver,
though Apple folks will need to change their TAPI allow list described
in the commit where these symbols were originally exported
https://github.com/llvm/llvm-project/commit/f5380185623be243ba0f1b18d4bd594ac5cc7163

Bug: https://github.com/llvm/llvm-project/issues/58265

Differential Revision: https://reviews.llvm.org/D135340