Relax assertion in ReplaceableMetadataImpl::replaceAllUsesWith().

There is a legitimate use-case in clang where we need to replace a
temporary placeholder node with the temporary node that may be a
forward declaration.

<rdar://problem/24493203>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259973 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Metadata.cpp

@@ -188,8 +188,6 @@ void ReplaceableMetadataImpl::moveRef(void *Ref, void *New,
 }
 
 void ReplaceableMetadataImpl::replaceAllUsesWith(Metadata *MD) {
-  assert(!(MD && isa<MDNode>(MD) && cast<MDNode>(MD)->isTemporary()) &&
-         "Expected non-temp node");
   assert(CanReplace &&
          "Attempted to replace Metadata marked for no replacement");