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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> Maybe it's time to bring back the ASTContext argument to Decl::getAttrs() etc?

We can potentially do this, it's just a lot of changes.  @joker-eph posted a partial patch for this: https://github.com/llvm/llvm-project/commit/f04bee1dc28a9ce82c6050ceb1f0831406e44380 .

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