[Sema] Don't skip function bodies with 'auto' without trailing return type
Skipping them was clearly not intentional. It's impossible to
guarantee correctness if the bodies are skipped.
Also adds a test case for r327504, now that it does not produce
invalid errors that made the test fail.
Reviewers: aaron.ballman, sammccall, rsmith
Reviewed By: rsmith
Subscribers: rayglover-ibm, rwols, cfe-commits
Differential Revision: https://reviews.llvm.org/D44480