Fixed failing assert in code completion.

Summary:
The code was accessing uninstantiated default argument.
This resulted in failing assertion at ParmVarDecl::getDefaultArg().

Reviewers: erikjv, klimek, bkramer, krasimir

Reviewed By: krasimir

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D35682

llvm-svn: 308722