[clang] [llvm] [KeyInstr] Fix verifier check (PR #149043)

[Stephen Tozer via cfe-commits]

SLTozer wrote:

So just to makes sure my understanding is correct: we have an existing bug and an existing test which should exercise that bug, but the verifier doesn't catch it currently. This patch then fixes both the verifier and the bug, and therefore a new test isn't needed for the non-verifier change because the coverage for the non-verifier change already exists in `coro-dwarf-key-instrs.cpp` - is that about right? And agreeing with the above, this should include a small test that the verifier check works.

https://github.com/llvm/llvm-project/pull/149043