Remove compile time PreserveName in favor of a runtime cc1 -discard-value-names option

Summary:
This flag is enabled by default in the driver when NDEBUG is set. It
is forwarded on the LLVMContext to discard all value names (but
GlobalValue) for performance purpose.

This an improved version of D18024

Reviewers: echristo, chandlerc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18127

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263394