Sema: Replace getAccessor() calls in SPI attribute checking

This call only succeeds if the accessor has already been synthesized, which
makes it fragile. Replace it with calls to supportsMutation(), or to
getOpaqueAccessor(), which lazily evaluate the right requests ahead of time.

Author: slavapestov

lib/Sema/TypeCheckAttr.cpp

@@ -897,8 +897,8 @@ void AttributeChecker::visitSPIAccessControlAttr(SPIAccessControlAttr *attr) {
           // implementation defines `set` if the protocol declares it.
           if (auto protoStorage = dyn_cast<AbstractStorageDecl>(VD))
             if (auto entryStorage = dyn_cast<AbstractStorageDecl>(entryDecl))
-              if (protoStorage->getAccessor(AccessorKind::Set) &&
-                  !entryStorage->getAccessor(AccessorKind::Set))
+              if (protoStorage->supportsMutation() &&
+                  !entryStorage->supportsMutation())
                 return false;
 
           return true;