[AST] Allow access-scope based access check inside inaccessible members

This aids expression diagnostics to pinpoint problems related to
chaining of inaccessible members  e.g.

```swift
struct A {
  struct B {}
}

_ = A.B()
```

Here both `B` and its synthesized constructor are inaccessible.

Author: xedin

lib/AST/Decl.cpp

@@ -2824,6 +2824,13 @@ static bool checkAccess(const DeclContext *useDC, const ValueDecl *VD,
   auto access = getAccessLevel();
   auto *sourceDC = VD->getDeclContext();
 
+  // Preserve "fast path" behavior for everything inside
+  // protocol extensions and operators, otherwise allow access
+  // check declarations inside inaccessible members via slower
+  // access scope based check, which is helpful for diagnostics.
+  if (!(sourceDC->getSelfProtocolDecl() || VD->isOperator()))
+    return checkAccessUsingAccessScopes(useDC, VD, access);
+
   if (!forConformance) {
     if (auto *proto = sourceDC->getSelfProtocolDecl()) {
       // FIXME: Swift 4.1 allowed accessing protocol extension methods that were
@@ -2878,21 +2885,8 @@ static bool checkAccess(const DeclContext *useDC, const ValueDecl *VD,
 
 bool ValueDecl::isAccessibleFrom(const DeclContext *useDC,
                                  bool forConformance) const {
-  bool result = checkAccess(useDC, this, forConformance,
-                            [&]() { return getFormalAccess(); });
-
-  // For everything outside of protocols and operators, we should get the same
-  // result using either implementation of checkAccess, because useDC must
-  // already have access to this declaration's DeclContext.
-  // FIXME: Arguably, we're doing the wrong thing for operators here too,
-  // because we're finding internal operators within private types. Fortunately
-  // we have a requirement that a member operator take the enclosing type as an
-  // argument, so it won't ever match.
-  assert(getDeclContext()->getSelfProtocolDecl() || isOperator() ||
-         result ==
-             checkAccessUsingAccessScopes(useDC, this, getFormalAccess()));
-
-  return result;
+  return checkAccess(useDC, this, forConformance,
+                     [&]() { return getFormalAccess(); });
 }
 
 bool AbstractStorageDecl::isSetterAccessibleFrom(const DeclContext *DC,