Conditionally drop the discriminant field in quirk_rust_enum

Conditionally drop the discriminant field in quirk_rust_enum

While debugging the crash that Jan reported, I noticed that in some
situations we could end up with a situation where one branch of a Rust
enum type ended up with a field count of -1.

The fix is simple: only conditionally drop the discriminant field when
rewriting the enum variants.

I couldn't find a way to test this; I only noticed it while debugging
the DWARF reader.

2018-04-17  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (quirk_rust_enum): Conditionally drop the
	discriminant field.