[clang] [llvm] [Clang][AST][NFC] Make Decl reference attributes directly (PR #219138)

[Steffen Larsen via llvm-commits]

steffenlarsen wrote:

Performance benefits can be found [here](https://llvm-compile-time-tracker.com/compare.php?from=7380050ae1a8261400350dd08a952ad3f00db1b3&to=a0af2dabaf4a5a63a8b4d04a5711b42ac0d39420&stat=instructions%3Au).

https://github.com/llvm/llvm-project/pull/219138