Print Rust unsized array types a bit more nicely

Print Rust unsized array types a bit more nicely

It's a bit difficult to create an unsized array type in Rust, but if
you do, right now ptype will show something like "[u8; ]".  It really
should print "[u8]", though, which is what this patch implements.

This is part of PR 21466.

Built and regtested on x86-64 Fedora 25.  I'm checking this in.

ChangeLog
2017-05-21  Tom Tromey  <tom@tromey.com>

	PR rust/21466:
	* rust-lang.c (rust_print_type) <TYPE_CODE_ARRAY>: Print unsized
	arrays as "[T]", not "[T; ]".

testsuite/ChangeLog
2017-05-21  Tom Tromey  <tom@tromey.com>

	PR rust/21466:
	* gdb.rust/unsized.exp: New file.
	* gdb.rust/unsized.rs: New file.