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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

In fact, I can delete `DirectoryCache`'s copy and move constructors and the code compiles, suggesting that the `StringMap` never copies or moves them.

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