Fix two latent Rust bugs

Fix two latent Rust bugs

Two methods on general_symbol_info did not handle the language_rust
case.  I don't think these problems can be noticed with the current
code (which is why the bugs went unnoticed), but a future patch will
change this.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tom@tromey.com>

	* symtab.c (general_symbol_info::natural_name)
	(general_symbol_info::demangled_name): Check for language_rust.