[PATCH] Fix DebugInfo replaceAllUsesWith.

[Frederic Riss]

You'd have to ask David if he remembers the details. I dug the commit history of this stuff, and he is the one that introduced it. 

The commits are about memory leaks, thus I believe it was related to getting read of node allocated with MDNode::getTemporary(). In the following commits of the forward decl work, I definitely use it this way. If I have a real definition to replace the temporary forward decl, then RAUW with that, otherwise, RAUW with itself and it turns the temporary node in a permanent one avoiding memory leaks.

http://reviews.llvm.org/D5326