Add range accessors for captures of a LambdaExpr.

Summary:
This adds LambdaExpr::captures(), LambdaExpr::explicit_captures()
and LambdaExpr::implicit_captures() as simple wrappers over the underlying
*_begin()/*_end() functions.

Reviewers: aaron.ballman

Differential Revision: http://reviews.llvm.org/D3926