[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/SwiftLangSupport.h

@@ -107,6 +107,7 @@ public:
   void updateSemaInfo(SourceKitCancellationToken CancellationToken);
 
   void removeCachedAST();
+  void cancelBuildsForCachedAST();
 
   ImmutableTextSnapshotRef getLatestSnapshot() const;