Revert "Fix PR34298 - Allow std::function with an incomplete return type."

This reverts commit r312890 because the test case fails to compile for
older versions of Clang that reject initializing a const object without
a user defined constructor.

Since this patch should go into 5.0.1, I want to keep it an atomic change,
and will re-commit it with a fixed test case.

llvm-svn: 312891