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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

It took me a while to realize that we forgot to change title and description after I decided to use `str.format`, so I reverted abf1d0bea04ab5d5ed1be3708ce1cd86707d5c8f and reapplied it as 0f7791e9d82d7d40981eb1a781a61c1e988997fd, where I took the liberty of changing the title and description to accurately reflect what the changes actually do. I hope it's accurate enough that I won't have to do this again.

@mikomikotaishi I preserved your authorship of the commit, but before any of your future contributions can be merged, you need to make your e-mail address public. See [LLVM Developer Policy](https://llvm.org/docs/DeveloperPolicy.html#email-addresses) for details.

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