[ADT][NFC] Use declval to suppress warning for nullptr use.

[fakepaper56]

Summary

The patch uses declval instead of nullptr to suppress warning for calling member
function by nullptr. Also the patch replace is_same<*>::value with is_same_v<*>.

[dexonsmith]

LGTM too.

llvm/unittests/ADT/RangeAdapterTest.cpp

 }
 
 TYPED_TEST(RangeAdapterRValueTest, HasRbegin) {
   static_assert(has_rbegin<TypeParam>::value, "rbegin() should be defined");
 }
 
 TYPED_TEST(RangeAdapterRValueTest, RangeType) {
   static_assert(
-      std::is_same<
-          decltype(reverse(*static_cast<TypeParam *>(nullptr)).begin()),
-          decltype(static_cast<TypeParam *>(nullptr)->rbegin())>::value,
+      std::is_same_v<decltype(reverse(std::declval<TypeParam>()).begin()),
+                     decltype(std::declval<TypeParam>().rbegin())>,
       "reverse().begin() should have the same type as rbegin()");
   static_assert(
-      std::is_same<
-          decltype(reverse(*static_cast<const TypeParam *>(nullptr)).begin()),
-          decltype(static_cast<const TypeParam *>(nullptr)->rbegin())>::value,
+      std::is_same_v<decltype(reverse(std::declval<const TypeParam>()).begin()),
+                     decltype(std::declval<const TypeParam>().rbegin())>,
       "reverse().begin() should have the same type as rbegin() [const]");
 }
 
 } // anonymous namespace