In Sema, give derived '==' definitions the module's DerivedFileUnit as their decl context instead of the more general Module, and give it a backreference to the nominal type for which it was derived.
In SILGen, visit the derived global decls associated with Clang-imported enums, and give them shared linkage. Part of fixing <rdar://problem/16264703>.
Swift SVN r14875
This names the implicit argument "oldValue". Whether we keep implicit arguments or not
is a subject of debate, tracked by rdar://16268361.
Swift SVN r14819
If an enum has no cases with payloads, make it implicitly Equatable and Hashable, and derive default implementations of '==' and 'hashValue'. Insert the derived '==' into module context wrapped in a new DerivedFileUnit kind, and arrange for it to be codegenned with the deriving EnumDecl by adding a 'DerivedOperatorDecls' array to NominalTypeDecls that gets visited at SILGen time.
Swift SVN r14471
