[Bug d/117832] Use CONSTRUCTOR_ZERO_PADDING_BITS in the D FE

[ibuclaw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117832

--- Comment #2 from Iain Buclaw <ibuclaw at gcc dot gnu.org> ---
Gave a little test of this flag. Doesn't look like it will be able to replace
all the uses where `memset(0)` is currently generated in the D front-end.

It probably doesn't harm to have both this and `-ftrivial-auto-var-init=zero`
set by default, keeping in the explicit memsets for now. At worst just double
zeroing, but I'd expect the middle-end to discard as necessary.