[clang] [clang][ASTImporter] Fix import of variable template redeclarations. (PR #72841)

[Balázs Kéri via cfe-commits]

balazske wrote:

I plan to fix import of `VarTemplateSpecializationDecl` in a different PR. The indicated assertion "Missing call to MapImported?" is likely to related to this part.

https://github.com/llvm/llvm-project/pull/72841