[clang] [clang-tools-extra] [clang] Implement CWG2413 (implicit `typename` in conversion operators) (PR #195207)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Update PR description with the relevant wording and P1787R6 description, so that it goes into a commit message when this is merged.

https://github.com/llvm/llvm-project/pull/195207