[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

lib/SIL/SILDeclRef.cpp

@@ -484,6 +484,12 @@ FuncDecl *SILDeclRef::getFuncDecl() const {
   return dyn_cast<FuncDecl>(getDecl());
 }
 
+bool SILDeclRef::isSetter() const {
+  if (!hasFuncDecl())
+    return false;
+  return getFuncDecl()->isSetter();
+}
+
 AbstractFunctionDecl *SILDeclRef::getAbstractFunctionDecl() const {
   return dyn_cast<AbstractFunctionDecl>(getDecl());
 }