Code completion should not ignore default parameters in functions.

Summary:
Inorder to display the default arguments we must process the
CK_Optional chunks of CodeCompletionString while creating the Signature.

We do not create placeholders for default arguments.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65866

llvm-svn: 368186