[Runtime] Ensure we *always* generate names for ObjC classes.

If something fails while trying to generate a name for an ObjC class,
we fall back to a generated name based on the metadata pointer.  This
ensures that we won't crash, and if the name turns up in the debugger
or somewhere it should be obvious that something went wrong.

rdar://107718586

Author: al45tair

stdlib/public/runtime/Metadata.cpp

@@ -3034,11 +3034,26 @@ static inline ClassROData *getROData(ClassMetadata *theClass) {
   return (ClassROData*)(theClass->Data & ~uintptr_t(SWIFT_CLASS_IS_SWIFT_MASK));
 }
 
+// This gets called if we fail during copyGenericClassObjcName().  Its job is
+// to generate a unique name, even though the name won't be very helpful if
+// we end up looking at it in a debugger.
+#define EMERGENCY_PREFIX "$SwiftEmergencyPlaceholderClassName"
+static char *copyEmergencyName(ClassMetadata *theClass) {
+  char *nameBuf = nullptr;
+  asprintf(&nameBuf,
+           EMERGENCY_PREFIX "%016" PRIxPTR,
+           (uintptr_t)theClass);
+  return nameBuf;
+}
+
 static char *copyGenericClassObjCName(ClassMetadata *theClass) {
   // Use the remangler to generate a mangled name from the type metadata.
   Demangle::StackAllocatedDemangler<4096> Dem;
 
   auto demangling = _swift_buildDemanglingForMetadata(theClass, Dem);
+  if (!demangling) {
+    return copyEmergencyName(theClass);
+  }
 
   // Remangle that into a new type mangling string.
   auto typeNode = Dem.createNode(Demangle::Node::Kind::TypeMangling);
@@ -3048,7 +3063,7 @@ static char *copyGenericClassObjCName(ClassMetadata *theClass) {
 
   auto mangling = Demangle::mangleNodeOld(globalNode, Dem);
   if (!mangling.isSuccess()) {
-    return nullptr;
+    return copyEmergencyName(theClass);
   }
   llvm::StringRef string = mangling.result();
 
@@ -3061,7 +3076,7 @@ static char *copyGenericClassObjCName(ClassMetadata *theClass) {
   size_t allocationSize = string.size() + 1;
   if (addSuffix)
     allocationSize += 1;
-  
+
   auto fullNameBuf = (char*)swift_slowAlloc(allocationSize, 0);
   memcpy(fullNameBuf, string.data(), string.size());