If back-deploying prior to the introduction of name mangling and
runtime support for invertible constraints (~Copyable, ~Escapable),
don't use mangled names to access metadata. The code already existed
for this, but had a carve-out that still used mangled names for
standard library types that have always existed but got generalized to
support non-copyable & non-escapable types.
Tweak that carve-out to not apply to types like Span that come from a
back-deployment library. Fixes crashes when using metadata for Span et
al on older platforms.
Fixes rdar://155639204.
