[libc++] Update GDB pretty-printer to work with GDB 17 (#142106)

This patch fixes an issue in libcxx/utils/gdb/libcxx/printers.py.

With gdb 17 (binutils 2_44) pretty-printers do not work anymore because
calls to `gdb.printing` requires `import gdb.printing` statement, which
was missing from the `printers.py`.

This was broken after commit https://github.com/bminor/binutils-gdb/commit/fc14343205d3a
and `import gdb.printing` was first referenced in https://github.com/bminor/binutils-gdb/commit/ee06c79b0f.

Co-authored-by: Dmitry Chestnykh <dm.chestnykh@gmail.com>

libcxx/utils/gdb/libcxx/printers.py

@@ -14,6 +14,7 @@ from __future__ import print_function
 
 import re
 import gdb
+import gdb.printing
 
 # One under-documented feature of the gdb pretty-printer API
 # is that clients can call any other member of the API