[clang] [clang][Python] Use fstrings instead of string concatenations (PR #173861)

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mikomikotaishi wrote:

I have noticed the code did use fstrings previously in a few locations, but string interpolation is generally a lesser cognitive load to read than the `str.format()` method which puts all the variables at the end. Personally I think it's easier to read the interpolated string with arguments in place.

https://github.com/llvm/llvm-project/pull/173861