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

[Mehdi Amini via cfe-commits]

joker-eph wrote:

What you have looks closer to some earlier versions of this PR I think, e.g. https://github.com/llvm/llvm-project/commit/99a9b8e86fdf154cee802677541c5a562cc3b36f ?

It grew to what it is now after the [comment here](https://github.com/llvm/llvm-project/pull/223047#discussion_r4065388840): 

> We might as well just past the ASTContext into the DeclContext constructor; should be simple to implement, and makes things slightly faster if we can skip doing a null check.

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