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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

Can you add two test for this in C++20 modules? One for it in module purview and one for it in GMF. 

My concern is, if this decl lives in GMF, it might be optimized out since no decl will reference it.

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