[Sema] Correct typos in return statements so the return types of 'auto'...

[Sema] Correct typos in return statements so the return types of 'auto' functions are always deduced.

Summary:
e.g.
  auto foo() {
    return no_such_thing; // Return value is a TypoExpr
  }
  using T = decltype(foo()); // Uh-oh, undeduced auto.

Reviewers: rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61649

llvm-svn: 360224