[clang] [Clang] Cache AST context lookup for declarations (NFC) (PR #223047)

[Nico Weber via cfe-commits]

nico wrote:

I have a similar change in much smaller here: https://github.com/llvm/llvm-project/pull/226612 (I wasn't aware of this PR here.)

> This is really trading speed for space, and not in a way that I'm really confident we want to do.

My approach has no measurable impact on peak RSS FWIW.

https://github.com/llvm/llvm-project/pull/223047