Made 'profiling' of objects in a FoldingSet trait-based using FoldingSetTrait

instead of always assuming that the stored objects had a method called
'Profile'. The default behavior is to dispatch to a 'Profile' method (as
before), but via template specialization this behavior can now be overridden by
clients.

Added templated class 'FoldingSetNodeWrapper', a generic wrapper class that
allows one to insert objects into a FoldingSet that do not directly inherit from
FoldingSetNode. This is useful for inserting objects that do not always need to
pay the overhead of inheriting from FoldingSetNode, or were designed with that
behavior in mind.

llvm-svn: 46186