[AST] Don’t deserialize members of @objc protocol looking for associated types

Thanks to @slavapestov for pointing this out! The optimization to avoid
looking through the members of a protocol that can’t possibly have any
associated types was for both deserialized protocols and imported
protocols, but I only supported the former in my previous change. Check
both cases and make the reasons much more obvious.

Also, that change resolved an existing compiler crasher as well.

Author: DougGregor

lib/AST/Decl.cpp

@@ -3651,13 +3651,22 @@ ProtocolDecl::getInheritedProtocols() const {
 llvm::TinyPtrVector<AssociatedTypeDecl *>
 ProtocolDecl::getAssociatedTypeMembers() const {
   llvm::TinyPtrVector<AssociatedTypeDecl *> result;
-  if (!hasClangNode()) {
-    for (auto member : getMembers()) {
-      if (auto ATD = dyn_cast<AssociatedTypeDecl>(member)) {
-        result.push_back(ATD);
-      }
+
+  // Clang-imported protocols never have associated types.
+  if (hasClangNode())
+    return result;
+
+  // Deserialized @objc protocols never have associated types.
+  if (!getParentSourceFile() && isObjC())
+    return result;
+
+  // Find the associated type declarations.
+  for (auto member : getMembers()) {
+    if (auto ATD = dyn_cast<AssociatedTypeDecl>(member)) {
+      result.push_back(ATD);
     }
   }
+
   return result;
 }
 

validation-test/compiler_crashers/28859-resolver-unable-to-resolve-type-witness.swift renamed to validation-test/compiler_crashers_fixed/28859-resolver-unable-to-resolve-type-witness.swift

@@ -5,6 +5,6 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+
+// RUN: not %target-swift-frontend %s -emit-ir
 class a:P@objc protocol P{{}func a:a{}{}typealias a