Refactored code so that the drop-flag values for initialized
(`DTOR_NEEDED`) versus dropped (`DTOR_DONE`) are given explicit names.

Add `mem::dropped()` (which with `DTOR_DONE == 0` is semantically the
same as `mem::zeroed`, but the point is that it abstracts away from
the particular choice of value for `DTOR_DONE`).

Filling-drop needs to use something other than `ptr::read_and_zero`,
so I added such a function: `ptr::read_and_drop`.  But, libraries
should not use it if they can otherwise avoid it.

Fixes to tests to accommodate filling-drop.

…ile-fail tests.

(I.e. the idea being, lets catch errors in these basic constructs
sometime *before* we start doing the doc tests.)

…ues.

…e PR.

(Reviewed rest of code; did not see other `pub` items that needed such
treatment.)

Driveby: fix typo in comment in ptr.rs.

(That is, added config and debugflag a la check-overflow but for drop
flag sanity-check.)

Remove now-unused import of NoDebugInfo from trans::glue.