[codeview] Workaround for PR43479, don't re-emit instr labels

Summary:
In the long run we should come up with another mechanism for marking
call instructions as heap allocation sites, and remove this workaround.
For now, we've had two bug reports about this, so let's apply this
workaround. SLH (the other client of instruction labels) probably has
the same bug, but the solution there is more likely to be to mark the
call instruction as not duplicatable, which doesn't work for debug info.

Reviewers: akhuang

Subscribers: aprantl, hiraditya, aganea, chandlerc, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69068

llvm-svn: 375137
(cherry picked from commit fc69ad09)