[AST] InitAccessors: init accessor properties without setters use special write strategy

Such properties could be mutated only within initializer context
so they can employ a special "init accessor" strategy.

(cherry picked from commit 1e6c919)

Author: xedin

lib/AST/Decl.cpp

@@ -2413,10 +2413,15 @@ getDirectReadAccessStrategy(const AbstractStorageDecl *storage) {
 static AccessStrategy
 getDirectWriteAccessStrategy(const AbstractStorageDecl *storage) {
   switch (storage->getWriteImpl()) {
-  case WriteImplKind::Immutable:
+  case WriteImplKind::Immutable: {
+    if (storage->hasInitAccessor())
+      return AccessStrategy::getAccessor(AccessorKind::Init,
+                                         /*dispatch=*/false);
+
     assert(isa<VarDecl>(storage) && cast<VarDecl>(storage)->isLet() &&
            "mutation of a immutable variable that isn't a let");
     return AccessStrategy::getStorage();
+  }
   case WriteImplKind::Stored:
     return AccessStrategy::getStorage();
   case WriteImplKind::StoredWithObservers:
@@ -2493,7 +2498,9 @@ getOpaqueReadAccessStrategy(const AbstractStorageDecl *storage, bool dispatch) {
 }
 
 static AccessStrategy
-getOpaqueWriteAccessStrategy(const AbstractStorageDecl *storage, bool dispatch){
+getOpaqueWriteAccessStrategy(const AbstractStorageDecl *storage, bool dispatch) {
+  if (storage->hasInitAccessor() && !storage->getAccessor(AccessorKind::Set))
+    return AccessStrategy::getAccessor(AccessorKind::Init, dispatch);
   return AccessStrategy::getAccessor(AccessorKind::Set, dispatch);
 }