Handle pointers and references correctly in DAP

A user pointed out that the current DAP variable code does not let the
client deference a pointer.  Oops!

Fixing this oversight is simple enough -- adding a new no-op
pretty-printer for pointers and references is quite simple.

However, doing this naive caused a regession in scopes.exp, which
expected there to be no children of a 'const char *' variable.  This
problem was fixed by the preceding patches in the series, which ensure
that a C type of this kind is recognized as a string.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30821