[ASTImporter] Add Visitor for TypedefNameDecl's

We found a case where Typedef Name Declarations were not being added
correctly when importing builtin types. This exposed the need for a
TypedefNameDecl visitor so these types can be added by RecordDecl and
fields.

This code is covered by the ASTImporterTest cases that use the implicit
struct __NSConstantString_tag definitions.

Thanks to @martong for the debugging assist!

Depends on D83970.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D83992