Fix crash in quirk_rust_enum
    
    I noticed that quirk_rust_enum can crash when presented with a union
    whose fields are all scalar types.
    
    This patch adds a new test case and fixes the bug.
    
    Regression tested on Fedora 26 x86-64.
    
    2018-04-17  Tom Tromey  <tom@tromey.com>
    
    	* dwarf2read.c (quirk_rust_enum): Handle unions correctly.
    
    2018-04-17  Tom Tromey  <tom@tromey.com>
    
    	* gdb.rust/simple.rs (Union): New type.
    	(main): New local "u".
    	* gdb.rust/simple.exp (test_one_slice): Add new test case.