Added the hasLoopVariable sub-matcher for forRangeStmt.

Summary:
This sub-matcher makes it possible to access directly the range-based for
loop variable: forRangeStmt(hasLoopVariable(anything()).bind(...)).
I've tried to re-generate the docs, but the diffs seem to include much more than
this change could cause, so I'd better leave docs update to someone who knows
the intended changes in the contents better.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2702