[clang] Support `constexpr` for some `ASTNodeKind` member functions

Add `constexpr` support for:

  * The `getFromNodeKind` factory function
  * `isSame`
  * `isNone`
  * `hasPointerIdentity`

This enables these functions to be used in SFINAE context for AST node
types.

Differential Revision: https://reviews.llvm.org/D135816