[SourceKit] Implement a new SourceKit request to generate the interface for a given type. rdar://27306890 (#3586)

This patch allows SourceKit to generate the interface for a given type specified by its mangled name.
Type interface is refined decl printing with type parameters localized and unusable members hidden.

Required field:
   "key.request": "source.request.editor.open.interface.swifttype"
   "key.usr": the mangled name of the given type.

tools/SourceKit/lib/SwiftLang/SwiftInterfaceGenContext.h

@@ -42,6 +42,11 @@ public:
                                             bool SynthesizedExtensions,
                                             Optional<StringRef> InterestedUSR);
 
+  static SwiftInterfaceGenContextRef
+    createForTypeInterface(swift::CompilerInvocation Invocation,
+                           StringRef TypeUSR,
+                           std::string &ErrorMsg);
+
   static SwiftInterfaceGenContextRef createForSwiftSource(StringRef DocumentName,
                                                           StringRef SourceFileName,
                                                           ASTUnitRef AstUnit,