[IRGen] Don't use mangled names for metadata including Span et al when back-deploying #83012
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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.
While I'm here, also fix
nonisolated(nonsending)function types, which have a similar issue because they were introduced in Swift 6.2.Fixes rdar://155639204.