[AST] Fix certain consteval assignment and comma operator issues with fixed-point types.

Summary:
Assignment and comma operators for fixed-point types were being constevaled as other
binary operators, but they need special treatment.

Reviewers: rjmccall, leonardchan, bjope

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73189