[clang-tidy] Don't check decltype return types in `readability-const-return-type`

The checker removes `const`s that are superfluos and badly affect
readability. `decltype(auto)`/`decltype(expr)` are often const-qualified, but
have no effect on readability and usually can't stop being const-qualified
without significant code change.

Fixes https://github.com/llvm/llvm-project/issues/52890

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D119470