Add two new classes: WeakVH and AssertingVH. These are both "ValueHandles",

which are effectively smart pointers to Value*'s.  They are both very light
weight and simple, and react to values being destroyed or being RAUW'd.

WeakVN does a best effort to follow a value around, including through RAUW 
operations and will get nulled out of the value is destroyed.  This is useful
for the eventual "metadata that references a value" work, because it is a
reference to a value that does not show up on its use_* list.

AssertingVH is a pointer that compiles down to a dumb raw pointer when 
assertions are disabled.  When enabled, it emits an assertion if the 
pointed-to value is destroyed while it is still being referenced.  This
is very useful for Maps and other things, and should have caught the recent
bugs in CallGraph and Reassociate, for example.

llvm-svn: 68149

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