Remove compile time PreserveName switch based on NDEBUG

Summary:
Following r263086, we are now relying on a flag on the Context to
discard Value names in release builds.

Reviewers: chandlerc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18024

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263257