Bill's change in r95336 broke empty aggregates embedded

in other types.  fix this by only bumping zero-byte globals
up to a single byte if the *entire global* is zero size,
fixing PR6340.

This also fixes empty arrays etc to be handled correctly,
and only does this on subsection-via-symbols targets (aka
darwin) which is the only place where this matters.

llvm-svn: 101879