Make Rust error message mention the field name
    
    I noticed a spot in rust-lang.c where the placeholder "foo" was used
    instead of the actual field name.  This patch fixes the bug.
    
    2018-09-13  Tom Tromey  <tom@tromey.com>
    
    	PR rust/23650:
    	* rust-lang.c (rust_evaluate_subexp): Use field name, not "foo".
    
    gdb/testsuite/ChangeLog
    2018-09-13  Tom Tromey  <tom@tromey.com>
    
    	PR rust/23650:
    	* gdb.rust/simple.exp: Add test for enum field access error.