Warnings for DDMD source

[Johan Engelen via Digitalmars-d]

I'd like to enable warnings for building DDMD  (-w, or -wi if 
there is too much debate).

The outputted warnings will depend on the D compiler used, but at 
least using "-wi" may improve code quality I think.

Right now, there are a bunch of switch case warnings, e.g.:
argtypes.d(296): Warning: switch case fallthrough - use 'goto 
case;' if intended
argtypes.d(304): Warning: switch case fallthrough - use 'goto 
case;' if intended
canthrow.d(149): Warning: switch case fallthrough - use 'goto 
default;' if intended
dcast.d(363): Warning: switch case fallthrough - use 'goto case;' 
if intended
dcast.d(377): Warning: switch case fallthrough - use 'goto case;' 
if intended
dcast.d(463): Warning: switch case fallthrough - use 'goto 
default;' if intended
dcast.d(604): Warning: switch case fallthrough - use 'goto case;' 
if intended

And there is a funny:
target.d(269): Warning: statement is not reachable
on the line:
269:          return null; // avoid warning

These warnings are easily fixed and (for me) improve legibility.

Let me know what you think. I'm happy to submit a PR. In fact, 
why don't I do it right now, while you debate about it.