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

[Victor Chernyakin via cfe-commits]

localspook wrote:

In this case P1787R6 says:
> My proposed regularization of conversion-type-id interpretation (which includes resolving [CWG2413](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2413)) in [“‘Down with template!’”](http://lists.isocpp.org/core/2019/05/6178.php) [...] [is] incorporated.

Searching for all instances of "conversion-type-id" or "conversion-function-id" in the paper, the only change I found that's relevant to this DR is adding *conversion-type-id* to the list of type-only contexts, i.e. the line @cor3ntin linked to. (there's wording cleanup around it, but that's the only functional change relevant to this DR).

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