[SemaOverload] Fixed crash on code completion

Summary:
The relevant failing assertion message is:
../tools/clang/lib/Sema/SemaInit.cpp:8411: PerformCopyInitialization(): Assertion `InitE && "No initialization expression?"' failed.

See the added test case for a repro.

Reviewers: bkramer, sammccall, ioeric, hokein

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D44300

llvm-svn: 327134