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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209679 91177308-0d34-0410-b5e6-96231b3b80d8