[profiling] Remove dead profile name vars after emitting name data

The profile name variables passed to counter increment intrinsics are
dead after we emit the finalized name data in __llvm_prf_nm. However, we
neglect to erase these name variables. This causes huge size increases
in the __TEXT,__const section as well as slowdowns when linker dead
stripping is disabled. Some affected projects are so massive that they
fail to link on Darwin, because only the small code model is supported.

Fix the issue by throwing away the name constants as soon as we're done
with them.

Differential Revision: https://reviews.llvm.org/D29921

llvm-svn: 295084