[Sema] PreCheck: Diagnose standalone self within init accessors

'self' within init accessor could only be used to refer to
properties listed in `initializes` and `accesses` attributes.

include/swift/AST/DeclContext.h

@@ -82,6 +82,7 @@ namespace swift {
   class SerializedDefaultArgumentInitializer;
   class SerializedTopLevelCodeDecl;
   class StructDecl;
+  class AccessorDecl;
 
 namespace serialization {
 using DeclID = llvm::PointerEmbeddedInt<unsigned, 31>;
@@ -457,6 +458,18 @@ public:
     return const_cast<DeclContext*>(this)->getInnermostMethodContext();
   }
 
+  /// Returns the innermost accessor context that belongs to a property.
+  ///
+  /// This routine looks through closure, initializer, and local function
+  /// contexts to find the innermost accessor declaration.
+  ///
+  /// \returns the innermost accessor, or null if there is no such context.
+  LLVM_READONLY
+  AccessorDecl *getInnermostPropertyAccessorContext();
+  const AccessorDecl *getInnermostPropertyAccessorContext() const {
+    return const_cast<DeclContext*>(this)->getInnermostPropertyAccessorContext();
+  }
+
   /// Returns the innermost type context.
   ///
   /// This routine looks through closure, initializer, and local function

include/swift/AST/DiagnosticsSema.def

@@ -7342,6 +7342,10 @@ ERROR(init_accessor_accesses_attribute_on_other_declaration,none,
 ERROR(init_accessor_property_both_init_and_accessed,none,
       "property %0 cannot be both initialized and accessed",
       (DeclName))
+ERROR(invalid_use_of_self_in_init_accessor,none,
+      "'self' within init accessors can only be used to reference "
+      "properties listed in 'initializes' and 'accesses'; "
+      "init accessors are run before 'self' is fully available", ())
 
 
 #define UNDEFINE_DIAGNOSTIC_MACROS