[clang][NFC] Expand some `auto`s and add another test for matcher `isExpandedFromMacro`.

Summary: Spells out some `auto`s explicitly and adds another test for the matcher `isExpandedFromMacro`.

Reviewers: aaron.ballman

Subscribers: gribozavr, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73975