[Support] Use an inherited constructor in DOTGraphTraits (NFC)

This patch simplifies DOTGraphTraits with an inherited constructor.
This way, we do not need to repeat the constructor signature or
manually forward constructor arguments.

Author: kazutakahirata

llvm/include/llvm/Support/DOTGraphTraits.h

@@ -164,7 +164,7 @@ struct DefaultDOTGraphTraits {
 ///
 template <typename Ty>
 struct DOTGraphTraits : public DefaultDOTGraphTraits {
-  DOTGraphTraits (bool simple=false) : DefaultDOTGraphTraits (simple) {}
+  using DefaultDOTGraphTraits::DefaultDOTGraphTraits;
 };
 
 } // End llvm namespace