gdbsupport: bump unordered_dense to 4.8.0

gdbsupport: bump unordered_dense to 4.8.0

We don't need anything in this release, but I think it doesn't hurt to
just stay up to date.  The new version has a new include file, stl.h.
To keep things clean and separated, move the imported files to a new
sub-directory.  This requires a small change in
gdb/check-include-guards.py, to be able to ignore the whole new
directory.

Change-Id: Ic8c5d0dd5ea8b6691c99975d6ca78f637175ef42
Approved-By: Tom Tromey <tom@tromey.com>