Disable all warnings in gdb.rust/traits.rs
    
    With rustc 1.37, I started seeing compiler warnings from the traits.rs
    test case:
    
        warning: trait objects without an explicit `dyn` are deprecated
    
    It seems to me that we generally do not want warnings in these test
    cases.  At some point, we'll probably have to patch traits.rs to use
    the "dyn" keyword; by that time I expect that all the Rust compilers
    in common use will support it.  In the meantime it seemed simplest to
    simply disable all warnings in this file.
    
    gdb/testsuite/ChangeLog
    2019-09-30  Tom Tromey  <tromey@adacore.com>
    
    	* gdb.rust/traits.rs: Disable all warnings.