[clang] [Clang] Set the decl context to the instantiated constructor when instantiating the explicit specifier (PR #78053)

[via cfe-commits]

https://github.com/cor3ntin commented:

I think it looks good but I'd like @erichkeane's input.

I also wonder whether we should look at the instantiated parent to determine whether the lambda is dependent, instead of just looking at the non-instantiated parent.

Might fix a few bugs and it's an approach i never considered before

https://github.com/llvm/llvm-project/pull/78053