Teach the related-id checker to ignore things from other buffers

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -2398,6 +2398,10 @@ private:
   }
 
   bool passId(CharSourceRange Range) {
+    // FIXME: Ignore things that don't come from this buffer.
+    if (!SourceMgr.getRangeForBuffer(BufferID).contains(Range.getStart()))
+      return !Cancelled;
+
     unsigned Offset = SourceMgr.getLocOffsetInBuffer(Range.getStart(),BufferID);
     Ranges.insert({Offset, Range.getByteLength()});
     return !Cancelled;