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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > 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.
> 
> After testing, I found that:
> 
> * Full BMI: The EIDs of both the GMF and the purview are present.
> * Reduced BMI: The EID of the GMF is optimized away, while the one in the purview is retained.

Yes. BTW, the "Reduced BMI" will be the default and the only supported one. The "Full BMI" will only be used by build system which supports two-phase compilations to build the object file. So, almost all BMIs will be "Reduced BMI". 

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