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

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16bit-ykiko 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.

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