libstdc++: Fix find_type helper to work consistently

The find_type helper function sometimes results in "class X::name" and
lookup for that fails. For more details see "Problem 1" in
https://gcc.gnu.org/pipermail/libstdc++/2021-March/052132.html and the
example at https://sourceware.org/bugzilla/show_bug.cgi?id=27510#c2

This patch replaces typ.unqualified() with typ.tag, which is never
qualified, and will never include the 'class' or 'struct' keywords.
Using the .tag attribute should be safe here because we know we are
looking at a class type and we've already used strip_typedefs().

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (find_type): Use tag attribute
	instead of unqualified() method.

libstdc++-v3/python/libstdcxx/v6/printers.py

@@ -85,8 +85,8 @@ except ImportError:
 def find_type(orig, name):
     typ = orig.strip_typedefs()
     while True:
-        # Strip cv-qualifiers.  PR 67440.
-        search = '%s::%s' % (typ.unqualified(), name)
+        # Use Type.tag to ignore cv-qualifiers.  PR 67440.
+        search = '%s::%s' % (typ.tag, name)
         try:
             return gdb.lookup_type(search)
         except RuntimeError: