[clang] [clang] print correct context for diagnostics suppressed by deduction (PR #125453)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> it's a permitted point of instantiation; we could try it and see if it works well enough in practice. We could also delay implicit definitions of special members like we do for template instantiations to reduce the impact.

Another possibility would be only saving the context up to the point of use of the decl, and then finishing the ancestors from there.

https://github.com/llvm/llvm-project/pull/125453