[clang] [clang][deps] Avoid `CompilerInvocation` copies (PR #205632)

[Ben Langmuir via cfe-commits]

https://github.com/benlangmuir commented:

Seeing this as an upstream change made me realize it would be good to have a couple of simple unit tests for `withCowRef` const/non-const and visitMutPaths. Otherwise LGTM.

https://github.com/llvm/llvm-project/pull/205632