[Frontend] Treat function with skipped body as definition

Summary:
This fixes an invalid warning about missing definition of a function when
parsing with SkipFunctionBodies=true

Reviewers: bkramer, sepavloff

Reviewed By: sepavloff

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D41189

llvm-svn: 320696