Value of `__cpp_constexpr` is incorrect for `-std=c++20`

[elbeno]

When compiling with -std=c++20, clang defines __cpp_constexpr to be 201907L, and it should be 202002L.

https://godbolt.org/z/G6v7EvEfe

Clearly clang has support for https://wg21.link/P1330.

So the value of __cpp_constexpr in C++20 should be 202002L as given by https://wg21.link/p2493 and https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#__cpp_constexpr

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Ben Deane (elbeno)

When compiling with `-std=c++20`, clang defines `__cpp_constexpr` to be `201907L`, and it should be `202002L`.

https://godbolt.org/z/G6v7EvEfe

Clearly clang has support for https://wg21.link/P1330.

So the value of __cpp_constexpr in C++20 should be 202002L as given by https://wg21.link/p2493 and https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#__cpp_constexpr