Fixed a crash in code completion.

Summary: The crash occured when FunctionDecl was parsed with an initializer.

Reviewers: bkramer, klimek, francisco.lopes

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D37382

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312788 91177308-0d34-0410-b5e6-96231b3b80d8