(De-)serialization for foreign error conventions.

Printing a module as Objective-C turns out to be a fantastic way to
verify the (de-)serialization of foreign error conventions, so
collapse the parsing-driving Objective-C printing test of throwing
methods into the general test for methods.

Swift SVN r27870

include/swift/Serialization/ModuleFile.h

@@ -660,6 +660,9 @@ public:
   GenericParamList *maybeReadGenericParams(DeclContext *DC,
                                      llvm::BitstreamCursor &Cursor,
                                      GenericParamList *outerParams = nullptr);
+
+  /// Reads a foreign error conformance from \c DeclTypeCursor, if present.
+  Optional<ForeignErrorConvention> maybeReadForeignErrorConvention();
 };
 
 } // end namespace swift