Fix Python pretty-printing bug in Rust

An upstream Rust bug notes notes that the Python pretty-printing
feature is broken for values that appear as members of certain types
in Rust.

The bug here is that some of the Rust value-printing code calls
value_print_inner, a method on rust_language.  This bypasses the
common code that calls into Python.

I'm checking this in.

gdb/ChangeLog
2021-05-14  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_language::val_print_struct)
	(rust_language::print_enum): Use common_val_print, not
	value_print_inner.

gdb/testsuite/ChangeLog
2021-05-14  Tom Tromey  <tom@tromey.com>

	* gdb.rust/pp.exp: New file.
	* gdb.rust/pp.py: New file.
	* gdb.rust/pp.rs: New file.