[C++20] Add consteval-specific semantic for functions

Summary:
Changes:
 - Calls to consteval function are now evaluated in constant context but IR is still generated for them.
 - Add diagnostic for taking address of a consteval function in non-constexpr context.
 - Add diagnostic for address of consteval function accessible at runtime.
 - Add tests

Reviewers: rsmith, aaron.ballman

Reviewed By: rsmith

Subscribers: mgrang, riccibruno, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63960