Define ContextualFoldingSet, which stores a context parameter to pass down to

the Profile method.  Currently this only works with the default FoldingSetTraits
implementation.

The point of this is to allow nodes to not store context values which are only
used during profiling.  A better solution would thread this value through the
folding algorithms, but then those would need to be (1) templated and
(2) non-opaque.

llvm-svn: 105819