Remove the ref/value inconsistency in filter_decl_iterator.

Remove the ref/value inconsistency in filter_decl_iterator.

filter_decl_iterator had a weird mismatch where both op* and op-> returned T*
making it difficult to generalize this filtering behavior into a reusable
library of any kind.

This change errs on the side of value, making op-> return T* and op* return
T&.

(reviewed by Richard Smith)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155808 91177308-0d34-0410-b5e6-96231b3b80d8