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

[Hans Wennborg via cfe-commits]

zmodem wrote:

Oh, I misspoke above. These PRs add a pointer to DeclContext, not Decl (of which there are a lot more), so it doesn't undo the savings from moving attribute lists to ASTContext.

That seems fine to me, though long-term it would be nice to get rid of these redundant ASTContext pointers.

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