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

[Owen Pan via cfe-commits]

owenca wrote:

Had we used `StringRef::contains` initially, would you still insist that it be replaced with a "more performant" local function? Please note that the code is not on a performance critical path, and the difference in time it takes to run the relevant unit tests for 100 times is within a few milliseconds on my system. For example:
```
time for i in {1..100}; do FormatTests --gtest_filter=FormatTestComments\.ReflowsComments* > /dev/null; done
```
Before:
```
real	0m1.244s
user	0m1.098s
sys	0m0.123s
```
After:
```
real	0m1.246s
user	0m1.102s
sys	0m0.121s
```
Runtime aside, it's bad for maintainability to hardcode the characters in `Blanks` in a switch statement when there's library function readily available.

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