[Driver][Darwin] Emit an error when using -pg on OS without support for it.

Instead of letting a program fail at runtime, emit an error during
compilation.

rdar://problem/12206955

Reviewers: dexonsmith, bob.wilson, steven_wu

Reviewed By: steven_wu

Subscribers: jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D57991

llvm-svn: 354084