[clang-tidy] Fix a -Wrange-loop-construct warning (#215775)

As a follow up to commit d4c991d34c8c8568c
([clang-tidy][NFC] Apply const-correctness for auto 1/N (#213839)) we
make sure to use a const reference when iterating of FixLocations.

As indicated by -Wrange-loop-construct this prevents a copy from type
'std::pair<clang::SourceLocation, bool> const'.

GitOrigin-RevId: 68f0da5431dec2de8aed7ca0e54792c98cb110c4