[clang-tools-extra] [clangd] Fix dangling references to StringMap's value. (PR #169339)

[John Porto via cfe-commits]

jpporto wrote:

> Nice! Can you please add a testcase?

I am unsure how to add a test for this -- the error is non-deterministic. Moreover, the original commit didn't add a test, so it is hard to base off of that. I am open to suggestions, though.

https://github.com/llvm/llvm-project/pull/169339