[Stdlib] Put $ at the end of the Swift module's generic class ObjC names.

This avoids name conflicts between generic classes in the Swift modules of the old and new stdlibs when loaded into the same process.

Author: mikeash

stdlib/public/runtime/Metadata.cpp

@@ -2204,9 +2204,24 @@ static void initGenericClassObjCName(ClassMetadata *theClass) {
 
   auto string = Demangle::mangleNodeOld(globalNode);
 
-  auto fullNameBuf = (char*)swift_slowAlloc(string.size() + 1, 0);
+  // If the class is in the Swift module, add a $ to the end of the ObjC
+  // name. The old and new Swift libraries must be able to coexist in
+  // the same process, and this avoids warnings due to the ObjC names
+  // colliding.
+  bool addSuffix = strncmp(string.c_str(), "_TtGCs", 6) == 0;
+
+  size_t allocationSize = string.size() + 1;
+  if (addSuffix)
+    allocationSize += 1;
+  
+  auto fullNameBuf = (char*)swift_slowAlloc(allocationSize, 0);
   memcpy(fullNameBuf, string.c_str(), string.size() + 1);
 
+  if (addSuffix) {
+    fullNameBuf[string.size()] = '$';
+    fullNameBuf[string.size() + 1] = '\0';
+  }
+
   auto theMetaclass = (ClassMetadata *)object_getClass((id)theClass);
 
   getROData(theClass)->Name = fullNameBuf;