Print void types correctly in Rust

Rust prefers to not specify the return type of a function when it is unit
(`()`). The type is also referred to as "void" in debuginfo but not in actual
usage, so we should never be printing "void" when the language is Rust.

2016-06-27  Manish Goregaokar  <manish@mozilla.com>

gdb/ChangeLog:
    * rust-lang.c (rust_print_type): Print unit types as "()"
    * rust-lang.c (rust_print_type): Omit return type for functions
    returning unit

gdb/testsuite/ChangeLog:
    * gdb.rust/simple.rs: Add test for returning unit in a function
    * gdb.rust/simple.exp: Add expectation for functions returning unit