[clang] [llvm] [clang][NFC] fold ExpressionTraits and TypeTraits helpers into Traits.h (PR #210838)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

> If possible I'd like to use the same name for the tablegen file and the utilities generated from it.
> 
> `BuiltinTraits` sounds good to me, that's a little bit clearer. Should I rename Traits.td to BuiltinTraits.td as well? (as a separate PR, it changes a lot of files)

SGTM!

https://github.com/llvm/llvm-project/pull/210838