Print type name when printing Rust slice

Print type name when printing Rust slice

The recent change to how unsized Rust values are printed included a
small regression from past behavior.  Previously, a slice's type would
be printed, like:

    (gdb) print slice
    $80 = &[i32] [3]

The patch changed this to just

    (gdb) print slice
    $80 = [3]

This patch restores the previous behavior.

Reviewed-By: Simon Marchi <simon.marchi@efficios.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30330
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31517