[clang] [Clang][AST] Fix ExplicitInstantiationDecl accessors for variable templates with tag types (PR #197856)

[Kim Gräsman via cfe-commits]

kimgr wrote:

@16bit-ykiko Thank you! Let's not block this PR either way. I'll play around with this a bit later.

https://github.com/llvm/llvm-project/pull/197856