[all-+0] Add a new convention for setters: DefaultSetterConventions.

In a situation where normal arguments are +0, we want setters to still take
normal parameters at +1 since in most cases setters will be taking a parameter
that is being forwarded into to be store into a field.

Since this doesn't actually change the current ParameterConvention for setter
normal arguments, this is NFC.

rdar://34222540

include/swift/SIL/SILDeclRef.h

@@ -227,6 +227,10 @@ struct SILDeclRef {
   bool isFunc() const {
     return kind == Kind::Func;
   }
+
+  /// True if the SILDeclRef references a setter function.
+  bool isSetter() const;
+
   /// True if the SILDeclRef references a constructor entry point.
   bool isConstructor() const {
     return kind == Kind::Allocator || kind == Kind::Initializer;