[Sema] Pop function scope when instantiating a func with skipped body

Summary:
By calling ActOnFinishFunctionBody(). Previously we were only calling
ActOnSkippedFunctionBody, which didn't pop the function scope.
This causes a crash when running on our internal code. No test-case,
though, since I couldn't come up with a small example in reasonable
time.

The bug was introduced in r321174.

Reviewers: bkramer, sammccall, sepavloff, aaron.ballman

Reviewed By: sammccall, aaron.ballman

Subscribers: aaron.ballman, cfe-commits

Differential Revision: https://reviews.llvm.org/D44439

llvm-svn: 327504