[Error] Unify +Asserts/-Asserts behavior for checked flags in Error/Expected<T>.

[Error] Unify +Asserts/-Asserts behavior for checked flags in Error/Expected<T>.

(1) Switches to raw pointer and bitmasking operations for Error payload.
(2) Always includes the 'unchecked' bitfield in Expected<T>, even in -Asserts.
(3) Always propagates checked bit status in move-ops for both classes, even in
    -Asserts.

This should allow debug programs to link against release libraries without
encountering spurious 'unchecked error' terminations.

Error checks still aren't verified in release mode so this doesn't introduce
any new control flow, but it does require new bit-masking ops in release mode
to preserve the flag values during move ops. I expect the overhead to be
minimal, but if we discover any corner cases where it matters we could fix
this by making flag propagation conditional on a new build option.

llvm-svn: 285426