[AST] Improve overflow diagnostics for fixed-point constant evaluation.

Summary:
Diagnostics for overflow were not being produced for fixed-point
evaluation. This patch refactors a bit of the evaluator and adds
a proper diagnostic for these cases.

Reviewers: rjmccall, leonardchan, bjope

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73188