[PATCH] Add observers to Input Graph

[Simon Atanasyan]

LGTM

================
Comment at: include/lld/Core/InputGraph.h:74
@@ +73,3 @@
+  /// being returned.
+  void registerObserver(std::function<void(File *)> &fn);
+
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Maybe it is better to use the `llvm::function_ref` class to be consistent with main LLVM code.

http://llvm.org/docs/ProgrammersManual.html#passing-functions-and-other-callable-objects

http://reviews.llvm.org/D3735