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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> but I don't recall how to start it (@Endilll ?)

Contact Nikita Popov (`nikic` on Discord), asking him to add your fork to LLVM compile time tracker. Then you push branches that start with `perf/`, and observe the results some hours after.

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