Add workaround for implicit copy constructor generation problem.

Some versions of Clang seem to generate a non-working implicit copy constructor
for `RemoteRef<BuiltinTypeDescriptor>`, which results in all the reflection tests
failing.  Fix by declaring it explicitly.

rdar://101240198

Author: al45tair

include/swift/Remote/MetadataReader.h

@@ -63,6 +63,11 @@ class RemoteRef {
   RemoteRef()
     : Address(0), LocalBuffer(nullptr) {}
 
+  // <rdar://99715218> Some versions of clang++ sometimes fail to generate the
+  // copy constructor for this type correctly - add a workaround
+  RemoteRef(const RemoteRef &other)
+    : Address(other.Address), LocalBuffer(other.LocalBuffer) {}
+
   /*implicit*/
   RemoteRef(std::nullptr_t _) : RemoteRef() {}