[C++20] Stop claiming full support for consteval (for the moment!)

During Clang 15, 3d2629dd claimed we
achieved full support for consteval in C++20. However, further testing
shows that Clang doesn't correctly handle all of the examples from
https://wg21.link/P1073R3 and has several other known issues that are
preventing us from defining the `__cpp_consteval` macro.

I think we should only claim Partial support for the moment. Once we
correct the major outstanding issues, then I think we should change the
status back to full support and define __cpp_consteval at the same time
(even if it's only to the 201811L value instead of the latest value
from C++2b). This helps users understand the support situation more
clearly.

Differential Revision: https://reviews.llvm.org/D144572