[SourceKit] Cancel in-flight builds on editor.close

When closing a document, cancel any in-flight
builds happening for it.

rdar://127126348

tools/SourceKit/lib/SwiftLang/SwiftASTManager.h

@@ -313,6 +313,7 @@ public:
                                       bool AllowInputs = true);
 
   void removeCachedAST(SwiftInvocationRef Invok);
+  void cancelBuildsForCachedAST(SwiftInvocationRef Invok);
 
   struct Implementation;
   Implementation &Impl;