If the feature is enabled, base the requirement for the underscored
accessors on the availability of the non-underscored accessors. If the
(non-underscored) accessor's was available earlier than the feature,
interpret that to mean that the underscored version was available in
that earlier version, and require the underscored version. The goal is
to ensure that the ABI is preserved, so long as the simplest migration
is done (namely, deleting the underscores from the old accessors).
For modify2, cache the required-ness in the same way that it is cached
for modify.
Not all runtimes can correctly operate with types that use noncopyable
generics. When the generic argument of a type is noncopyable, old
runtimes can't recognize that to correctly check conformances that may
be conditional on those arguments being Copyable, etc.
resolves rdar://126239335
and implement it for @isolated(any) function types.
The existing testing was pretty broken: we were diagnosing all sorts
of things that don't require type metadata (like using a tuple with
an extended existential in a value position in an API signature)
and not diagnosing several things that do (like covariant function
conversions that erase types). There's therefore some risk to this
patch, but I'm not too worried because needing metadata like this is
pretty uncommon, and it's likely that programs won't build correctly
anyway --- it'll just get caught by the linker instead of the compiler.
We need to only generate references to `_swift_exceptionPersonality`
if we're building for a new enough runtime. The previous code was
good on Darwin, but would have resulted in build problems on Linux.
rdar://120952971
