[clang] [Clang][NFC] Rename SecondArgIsLastNamedArgument for clarity and consistency (PR #131346)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I'm in favor, but lets let @AaronBallman take a look/click merge if is OK too.

https://github.com/llvm/llvm-project/pull/131346