Store State objects by value in TableGen's DFAPacketizerEmitter

Removes some extra manual dynamic memory allocation/management. It does
get a bit quirky having to make State's members mutable and
pointers/references to const rather than non-const, but that's a
necessary workaround to dealing with the std::set elements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206807 91177308-0d34-0410-b5e6-96231b3b80d8