[include-fixer] Correct an incorrecst judgement about prefix scoped qualifiers.

Summary:
The judgement that checks whether the fully-qualified name has scoped qualifiers
prefix is incorrect. Should always check whether the first matched postion is the
beginning position.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D22343

llvm-svn: 275386