GDB pretty printers: Basic DenseMap support

Still prints the empty/tombstone keys (which some people would prefer,
but I find pretty noisy) because I haven't yet found a reliable way to
skip them (it requires calling into the running process to do so, which
isn't ideal for a pretty printer (doesn't work on a core file, for
example) - and gdb's ability to do so (or my ability to figure out how
to get gdb to do so) is limited) left some breadcrumbs for the next
person who might try to address that.

llvm-svn: 290011