[clang] [clang-tools-extra] [Clang][AST] Introduce `ExplicitInstantiationDecl` to preserve source info and fix diagnostic locations (PR #191658)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I think this is a good direction, we should probably alert our downstreams @llvm/clang-vendors to make sure they are comfortable with the new node/having to deal with the new node in AST searches.

We should probably make sure our AST-matchers has a way of seeing these?

Also: I'm a bit concerned about the additional memory pressure, the `ExplicitInstantiationDecl` is pretty huge.  Can we measure the difference in memory usage in some sort of major project here to see?  

I'm also interested in if this has a sizable negative effect on compile times, we have the tracker, but I don't recall how to start it (@Endilll ?).

https://github.com/llvm/llvm-project/pull/191658