[clang][NFC] Refactor coroutine_traits lookup

To allow transition from the TS-specified
std::experimental::coroutine_traits to the C++20-specified
std::coroutine_traits, we lookup in both places and provide helpful
diagnostics. This refactors the code to avoid separate paths to
std::experimental lookups.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D116029