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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

The wording is here https://eel.is/c++draft/temp.res.general#4.3.3. I think the change is correct.
P1787 made other changes to conversion functions but they affect caller-side lookup. 
Ergo, the changes in this PR are necessary and sufficient to address CWG2413.

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