Add specialization of FoldingSetTrait for std::pair.

[klimek]

Summary

Needed for clang's static analyzer. Jordan suggested to add it directly
in FoldingSet.h.

[jordan_rose]

Hm. This doesn't look like it will actually handle pairs containing any non-struct type, where you have to use AddInteger or AddPointer as appropriate. (Why don't those have FoldingSetTraits directly?)

[jordan_rose]

It looks like there's already a specialization for T*, so it's just the integers that get left out. Someone can add those later if they need them.

[klimek]

Thx, submitted as r210990.

include/llvm/ADT/FoldingSet.h

 //===----------------------------------------------------------------------===//
 // Partial specializations of FoldingSetTrait.
 
 template<typename T> struct FoldingSetTrait<T*> {
   static inline void Profile(T *X, FoldingSetNodeID &ID) {
     ID.AddPointer(X);
   }
 };
+template <typename T1, typename T2>
+struct llvm::FoldingSetTrait<std::pair<T1, T2>> {
+  static inline void Profile(const std::pair<T1, T2> &P,
+                             llvm::FoldingSetNodeID &ID) {
+    ID.Add(P.first);
+    ID.Add(P.second);
+  }
+};
 } // End of namespace llvm.
 
 #endif