swift234

Sema: Always synthesize accessors for structs, unless they were imported from Clang

This fixes the issue that "SILGen: Correctly emit accessors synthesized to
witness protocol requirements" was meant to solve, but in a simpler way.

A better fix would be to first address the issue where @_transparent
function bodies are not serialized in some cases, and then only emit
synthesized accessors as needed, in the original version of this patch.
To fix the duplicate symbol issues, we would emit the synthesized
accessors with shared linkage, which would always work once serialized
bodies were available.

For resilient structs of course, we'll always need to emit accessors
anyway.