[CodeComplete] Fix completion in the middle of idents in macro calls

Summary:
This patch removes IdentifierInfo from completion token after remembering
the identifier in the preprocessor.

Prior to this patch, completion token had the IdentifierInfo set to null when
completing at the start of identifier and to the II for completion prefix
when in the middle of identifier.

This patch unifies how code completion token is handled when it is insterted
before the identifier and in the middle of the identifier.

The actual IdentifierInfo can still be obtained from the Preprocessor.

Reviewers: bkramer, arphaman

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42241

llvm-svn: 323133