[clang] [clang][ASTImporter] Improve import of variable template specializations. (PR #78284)

[Balázs Kéri via cfe-commits]

balazske wrote:

After this change it should be possible to add a
```
static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
                                     VarTemplateDecl *D1, VarTemplateDecl *D2)
```
function with all tests passing.

https://github.com/llvm/llvm-project/pull/78284