Add reverse(ContainerTy) range adapter.

For cases where we needed a foreach loop in reverse over a container,
we had to do something like

  for (const GlobalValue *GV : make_range(TypeInfos.rbegin(),
                                          TypeInfos.rend())) {

This provides a convenience method which shortens this to

  for (const GlobalValue *GV : reverse(TypeInfos)) {

There are 2 versions of this, with a preference to the rbegin() version.

The first uses rbegin() and rend() to construct an iterator_range.

The second constructs an iterator_range from the begin() and end() methods
wrapped in std::reverse_iterator's.

Reviewed by David Blaikie.

llvm-svn: 243563