Rollup merge of rust-lang#116985 - tromey:rust-printers-14, r=Mark-Simulacrum

Use gdb.ValuePrinter tag class

GDB 14 has a "gdb.ValuePrinter" tag class that was introduced to let GDB evolve the pretty-printer API.  Users of this tag are required to hide any local attributes or methods.  This patch makes this change to the Rust pretty-printers.  At present this is just a cleanup, providing the basis for any future changes.

Author: matthiaskrgr