[clang] [clang-format][NFC] Replace a function with StringRef::contains (PR #146245)

[Björn Schäpers via cfe-commits]

HazardyKnusperkeks wrote:

> Had we used `StringRef::contains` initially, would you still insist that it be replaced with a "more performant" local function? 

Of course not, because no one would go look for it.

https://github.com/llvm/llvm-project/pull/146245