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

gdb/python/lib/gdb/printing.py

@@ -280,6 +280,20 @@ class NoOpScalarPrinter:
         return self.value.format_string(raw=True)
 
 
+class NoOpPointerReferencePrinter:
+    """A no-op pretty printer that wraps a pointer or reference."""
+
+    def __init__(self, value):
+        self.value = value
+        self.num_children = 1
+
+    def to_string(self):
+        return self.value.format_string(deref_refs=False)
+
+    def children(self):
+        yield "value", self.value.referenced_value()
+
+
 class NoOpArrayPrinter:
     """A no-op pretty printer that wraps an array value."""
 
@@ -354,6 +368,8 @@ def make_visualizer(value):
             result = gdb.printing.NoOpArrayPrinter(ty, value)
         elif ty.code in (gdb.TYPE_CODE_STRUCT, gdb.TYPE_CODE_UNION):
             result = gdb.printing.NoOpStructPrinter(ty, value)
+        elif ty.code in (gdb.TYPE_CODE_PTR, gdb.TYPE_CODE_REF, gdb.TYPE_CODE_RVALUE_REF):
+            result = NoOpPointerReferencePrinter(value)
         else:
             result = gdb.printing.NoOpScalarPrinter(value)
     return result