Emit label names according to -discard-value-names.

Summary:
Previously, Clang only emitted label names in assert builds.
However there is a CC1 option -discard-value-names that should have been used to control emission instead.
    
This patch removes the NDEBUG preprocessor block and instead allows LLVM to handle removing the names in accordance with the option.


Reviewers: erichkeane, aaron.ballman, majnemer

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42829

llvm-svn: 324127