[clang] [Sema] Fix crash on invalid operator template-id (PR #181404)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Nah, it is perfectly reasonable code ! https://compiler-explorer.com/z/4Y3Wfcx79 @ojhunt

Well that's horrifying, but it's more clear inside a decltype that a typedef :D

I think including the simpler test case makes sense though - you don't really need the paraphernalia not relevant to triggering it (lest people think it's important)

https://github.com/llvm/llvm-project/pull/181404