swift234

Sema: Better fix for duplicate emission of accessors

When compiling a module with multiple FileUnits (eg, while running
swiftc without -whole-module-optimization), we emit all decls
in the ExternalDeclarations list for every FileUnit, leading to
duplicate symbols if any of those decls were public.

Joe's workaround skipped decls on ExternalDefinitions that were
already emitted in the current FileUnit, but that was not sufficient;
the right fix is to not put stuff on ExternalDeclarations at all,
if we're going to emit it as part of a decl inside our file.

In this case, the synthesized _code accessors for an ErrorType
conformance do not belong in ExternalDefinitions, because they're
part of an ExtensionDecl that is in the current FileUnit.

Finally add some tests for this stuff, because it appears our
existing coverage was insufficient.