[clang-tidy] Fix potential assert in use-noexcept check

Summary: Fix a potential assert in use-noexcept check if there is an issue getting the `TypeSourceInfo` as well as a small clean up.

Reviewers: aaron.ballman, alexfh, gribozavr2

Reviewed By: aaron.ballman

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80371