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

[Raphael Isemann via cfe-commits]

Teemperor wrote:

I don't see any concerns with regard to LLDB's internal ASTs. The Decls there belong to only one ASTContext and are recreated if they get moved anywhere else.

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