[WebAssembly] Improve WasmSignatureDenseMapInfo.

[ruiu]

Summary

Let X and Y be types. Previously, functions F(X, Y) and G(Y, X) had
the same hash value because their hash values are computed as follows:

hash(F) = hash(X) + hash(Y)
hash(G) = hash(Y) + hash(X)

This patch fixes the issue by using hash_combine.

wasm/Writer.cpp

static WasmSignature getEmptyKey() {
     return Sig;
   }
   static WasmSignature getTombstoneKey() {
     WasmSignature Sig;
     Sig.ReturnType = 2;
     return Sig;
   }
   static unsigned getHashValue(const WasmSignature &Sig) {
-    uintptr_t Value = 0;
-    Value += DenseMapInfo<int32_t>::getHashValue(Sig.ReturnType);
+    unsigned H = hash_value(Sig.ReturnType);
     for (int32_t Param : Sig.ParamTypes)
-      Value += DenseMapInfo<int32_t>::getHashValue(Param);
-    return Value;
+      H = hash_combine(H, Param);
+    return H;
   }
   static bool isEqual(const WasmSignature &LHS, const WasmSignature &RHS) {
     return LHS == RHS;
   }
 };
 
 // An init entry to be written to either the synthetic init func or the
 // linking metadata.