[ASTImporter] Support functions with placeholder return types ...

Summary:
Support functions with placeholder return types even in cases when the type is
declared in the body of the function.
Example: auto f() { struct X{}; return X(); }

Reviewers: balazske, a_sidorin, a.sidorin, shafik

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, teemperor, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70819