[llvm] r271669 - Adding reserve and capacity methods to FoldingSet

[Craig, Ben via llvm-commits]

> You may be able to mimic "struct CountCopyAndMove" and the associated 
> tests in unittests/ADT/DenseMapTest.cpp
FoldingSet doesn't copy or move it's elements on a rehash.  It just 
splices them into a different singly linked list.  There isn't currently 
a user hook for telling when an item is relinked.  You could check a 
node before and after a resize operation to see if the link changed, but 
you need to generate a bucket collision first.

> Doxygen on public API
Will do.

> I was surprised that we target a load factor >1, but I'm still unsure 
> about the right use-case for FoldingSet compare to a DenseSet for 
> instance. Is it about consuming less memory?
DenseSet contains and owns the elements.  It works best with small keys 
and values, because it can be a lot nicer about cache locality.  
FoldingSet works better with large key/value pairs, because in those 
situations you know you're going to get some cache misses anyway.

I'm unclear on the rationale for the high load factor, but I didn't 
change that in this review.

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