[CodeComplete] Fix completion in the middle of ident in ctor lists.

Summary:
The example that was broken before (^ designates completion points):

    class Foo {
      Foo() : fie^ld^() {} // no completions were provided here.
      int field;
    };

To fix it we don't cut off lexing after an identifier followed by code
completion token is lexed. Instead we skip the rest of identifier and
continue lexing.
This is consistent with behavior of completion when completion token is
right before the identifier.

Reviewers: sammccall, aaron.ballman, bkramer, sepavloff, arphaman, rsmith

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D44932

llvm-svn: 330833