[C++11] Adding an iterator_range accessor for parameter declarations to the CaptureDecl class; however, the only usage of the iterators cannot be converted to using the range yet. This change is for consistency with other Decls exposing parameters.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203252 91177308-0d34-0410-b5e6-96231b3b80d8