[AST] Use std::bool_constant instead of std::integral_constant (NFC)

This patch replaces, std::integral_constant<bool, ...> with
std::bool_constant for brevity.  Note that std::bool_constant was
introduced as part of C++17.

While I am at it, this patch uses the "_v" variants of type traits.

Author: kazutakahirata

clang/include/clang/AST/TypeBase.h

@@ -9093,9 +9093,8 @@ inline const StreamingDiagnostic &operator<<(const StreamingDiagnostic &PD,
 // Helper class template that is used by Type::getAs to ensure that one does
 // not try to look through a qualified type to get to an array type.
 template <typename T>
-using TypeIsArrayType =
-    std::integral_constant<bool, std::is_same<T, ArrayType>::value ||
-                                     std::is_base_of<ArrayType, T>::value>;
+using TypeIsArrayType = std::bool_constant<std::is_same_v<T, ArrayType> ||
+                                           std::is_base_of_v<ArrayType, T>>;
 
 // Member-template getAs<specific type>'.
 template <typename T> const T *Type::getAs() const {