[OPENMP] Delay emission of the asm target-specific error messages.

Summary:
Added the ability to emit target-specific builtin assembler error
messages only in case if the function is really is going to be emitted
for the device.

Reviewers: rjmccall

Subscribers: guansong, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58243

llvm-svn: 354486