RFC: constify ada_decode_symbol

[Joel Brobecker]

> ada_decode_symbol can be a little more const-correct.
> It still needs a cast, due to how it writes into 'gsymbol', but the
> resulting API is better, as it exposes more const-ness to the caller.
> 
> Tested by rebuilding.
> 
> Tom
> 
> 	* ada-lang.h (ada_decode_symbol): Make return type const.
> 	* ada-lang.c (ada_decode_symbol): Likewise.

Thanks, Tom. I also think this is better, so please feel free to
go ahead and commit. I think I have another patch that adds constness
that's in our tree, and I haven't checked yet whether it could be
applied to the FSF tree. I'll try to work on that next week.

Cheers,
-- 
Joel