Sema: Synthesize accessors for computed local and global properties

If a stored property has a didSet/willSet, we currently synthesize
the getter and the setter inside the parser.  I want to get rid of
the parser's code path for doing that and consolidate all accessor
synthesis here in Sema.

Note that the parser did not synthesize a modify. This is now more
explicit in the code.

Also, we have to relax an assertion since addMemberToContextIfNeeded()
now gets called on declarations in local contexts.

Author: slavapestov

lib/AST/Decl.cpp

@@ -1907,6 +1907,16 @@ bool AbstractStorageDecl::requiresOpaqueModifyCoroutine() const {
   if (!supportsMutation())
     return false;
 
+  auto *dc = getDeclContext();
+
+  // Local properties don't have modify accessors.
+  if (dc->isLocalContext())
+    return false;
+
+  // Fixed-layout global properties don't have modify accessors.
+  if (dc->isModuleScopeContext() && !isResilient())
+    return false;
+
   // Imported storage declarations don't have eagerly-generated modify
   // accessors.
   if (hasClangNode())
@@ -1919,7 +1929,6 @@ bool AbstractStorageDecl::requiresOpaqueModifyCoroutine() const {
     return false;
 
   // Requirements of ObjC protocols don't support the modify coroutine.
-  auto *dc = getDeclContext();
   if (auto protoDecl = dyn_cast<ProtocolDecl>(dc))
     if (protoDecl->isObjC())
       return false;

lib/Sema/CodeSynthesis.cpp

@@ -72,7 +72,7 @@ static void addMemberToContextIfNeeded(Decl *D, DeclContext *DC,
   } else if (auto *ed = dyn_cast<ExtensionDecl>(DC)) {
     ed->addMember(D, Hint);
   } else {
-    assert((isa<AbstractFunctionDecl>(DC) || isa<FileUnit>(DC)) &&
+    assert((DC->isLocalContext() || isa<FileUnit>(DC)) &&
            "Unknown declcontext");
   }
 }
@@ -2036,8 +2036,8 @@ void swift::maybeAddAccessorsToStorage(AbstractStorageDecl *storage) {
 
   auto *dc = storage->getDeclContext();
 
-  // Local variables don't otherwise get accessors.
-  if (dc->isLocalContext())
+  // Local stored variables don't otherwise get accessors.
+  if (dc->isLocalContext() && storage->getImplInfo().isSimpleStored())
     return;
 
   // Implicit properties don't get accessors.
@@ -2053,7 +2053,7 @@ void swift::maybeAddAccessorsToStorage(AbstractStorageDecl *storage) {
       return;
     }
     // Fixed-layout global variables don't get accessors.
-    if (!storage->isResilient())
+    if (!storage->isResilient() && storage->getImplInfo().isSimpleStored())
       return;
 
   // In a protocol context, variables written as just "var x : Int" or