[clang][NFC] Make various uses of Regex const

The const-correctness of match() was fixed in rL372764, which allows
uses of Regex objects to be const in cases they couldn't be before. This
patch tightens up the const-ness of Regex in various such cases.

Reviewers: thopre

Reviewed By: thopre

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68155