[clang][ASTImporter] Import typedefs to distinct records as distinct nodes.

When a typedef node is imported, ASTImporter should not find an existing similar
typedef node for it that comes from different context (translation unit or scope).
This should avoid a situation where an existing typedef declaration is returned
at import of a typedef, but the underlying type was already imported as a new
type object.

Reviewed By: vabridgers

Differential Revision: https://reviews.llvm.org/D145479