swift234

SILGen: Correctly emit accessors synthesized to witness protocol requirements

We weren't adding them as external decls unless they were for
storage on an imported type, which meant SILGen wasn't emitting
them if the conforming type was from a different Swift source
file, or in whole-module mode, a different module. This led
to linker errors.

Instead, always add accessors to the external decl list, but
skip them in SILGen if they are contained in the DeclContext
we are currently emitting (which is a source file or module).

Note that they are still emitted with the wrong linkage, from a
resilience perspective. Clients must only ever see public
exports for getters, setters and materializeForSet emitted
because they are required by resilience or the access pattern;
'accidental' accessors synthesized for protocol conformance
should not be public.