ADT: Fix SmallPtrSet iterators in reverse mode

Fix SmallPtrSet::iterator behaviour and creation ReverseIterate is true.

  - Any function that creates an iterator now uses
    SmallPtrSet::makeIterator, which creates an iterator that
    dereferences to the given pointer.

  - In reverse-iterate mode, initialze iterator::End with "CurArray"
    instead of EndPointer.

  - In reverse-iterate mode, the current node is iterator::Buffer[-1].
    iterator::operator* and SmallPtrSet::makeIterator are the only ones
    that need to know.

  - Fix the assertions for reverse-iterate mode.

This fixes the tests Danny B added in r297182, and adds a couple of
others to confirm that dereferencing does the right thing, regardless of
how the iterator was found, and that iteration works correctly from each
return from find.

llvm-svn: 297234