This is a temporary workaround for some situations where the empty
singleton is not being formed correctly (and this seems to be highly
configuration dependent). Work around that for now for also checking
for empty non-storage-backed Strings.
This is probably too expensive a check for us to do long-term, but it
works for now.
Include the initial implementation of _StringGuts, a 2-word
replacement for _LegacyStringCore. 64-bit Darwin supported, 32-bit and
Linux support in subsequent commits.
