[clang] [clang][ASTImporter] set nonnull type for var specialized from lambda template (PR #105492)

[Balázs Kéri via cfe-commits]

balazske wrote:

This fix looks good, but I can not tell now why it would not work to import the type just before construction of the `VarTemplateSpecializationDecl`. I tried it and all tests passed. Probably I can test this on a set of projects.

https://github.com/llvm/llvm-project/pull/105492