[clang] [Clang] Cache DeclContext-to-Decl conversions (NFC) (PR #223042)

[Mehdi Amini via cfe-commits]

joker-eph wrote:

> Also, the benefit of this probably goes down if you combine it with #223047.

I measured this as well (Eager pointer  vs Tagged offset) and we retain non-trivial gains over  #223047 in both cases:

- RegisterAllDialects instructions           −2.22%                  vs   −1.66%
- NVVMDialect instructions                   −1.36%                   vs  −0.99%

https://github.com/llvm/llvm-project/pull/223042