[sancov][NFC] Make filename Regexes "const"

Summary:
The const-correctness of match() was fixed in rL372764, which allows
such static Regex objects to be marked const.

Reviewers: thopre

Reviewed By: thopre

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68091

llvm-svn: 373058