objc: clang should warn if redeclaration of methods

declared in protocol in the class qualified by the
protocol have type conflicts. To reduce amount of
noise, this is done when class is implemented.
// rdar://9352731


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135890 91177308-0d34-0410-b5e6-96231b3b80d8