Avoid crash on single-field union in Rust

PR rust/24976 points out a crash in gdb when a single-field union is
used in Rust.

The immediate problem was a NULL pointer dereference in
quirk_rust_enum.  However, that code is also erroneously treating a
single-field union as if it were a univariant enum.  Looking at the
output of an older Rust compiler, it turns out that univariant enums
are distinguished by having a single *anonymous* field.  This patch
changes quirk_rust_enum to limit its fixup to this case.

Tested with a new-enough version of the Rust compiler to cause the
crash; plus by using an older executable that uses the old univariant
encoding.

gdb/ChangeLog
2019-10-03  Tom Tromey  <tom@tromey.com>

	PR rust/24976:
	* dwarf2read.c (quirk_rust_enum): Handle single-element unions.

gdb/testsuite/ChangeLog
2019-10-03  Tom Tromey  <tom@tromey.com>

	PR rust/24976:
	* gdb.rust/simple.rs (Union2): New type.
	(main): Use Union2.
	* gdb.rust/simple.exp: Add test.