Remove ref/value inconsistency in redecl_iterator.

Similar to r155808 - this mistake has been made in a few iterators.

Based on Chandler Carruth's feedback to r155808 I added an implicit conversion
to Decl* to ease adoption/usage. Useful for the pointer comparison, but not the
dyn_cast (due to template argument deduction causing the conversion not to be
used) - there for future convenience, though. This idiom (op T* for iterators)
seems to be fairly idiomatic within the LLVM codebase & I'll likely add it as I
fix up the other iterators here.

llvm-svn: 155869