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

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

It would also be worth adding support for ast-print with a few tests, that looks fairly simple for this declaration.

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