[clang][ASTImporter] Improve import of functions with auto return type.

ASTImporter used to crash in some cases when a function is imported with
`auto` return type and the return type has references into the function.
The handling of such cases is improved and crash should not occur any more
but it is not fully verified, there are very many different types of
cases to care for.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D130705