Clear rust_unions in rust_union_quirks

It turns out that a dwarf2_cu can remain allocated after psymtab
expansion is done, and so it makes sense to clear rust_unions when
done processing it.

Tested on x86-64 Fedora 27.

2018-05-14  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (rust_union_quirks): Clear rust_unions.