[SourceKit] Accept swift_version in requests and use to set the swift version for interface generation of an ObjC header

rdar://31432960

tools/SourceKit/include/SourceKit/Core/LangSupport.h

@@ -443,7 +443,8 @@ public:
                                          StringRef Name,
                                          StringRef HeaderName,
                                          ArrayRef<const char *> Args,
-                                         bool SynthesizedExtensions) = 0;
+                                         bool SynthesizedExtensions,
+                                         Optional<unsigned> swiftVersion) = 0;
 
   virtual void editorOpenSwiftSourceInterface(StringRef Name,
                                               StringRef SourceName,