[Clang] Bump __cpp_constexpr to 202002L in C++20 mode (#129814)

Per P2493R0 and SD6, `__cpp_constexpr` of value `202002L` indicates that
P1330R0 "Changing the active member of a union inside constexpr" is
implemented, which is true for Clang 9 and later.

Author: frederick-vs-ja

clang/docs/ReleaseNotes.rst

@@ -96,6 +96,9 @@ Resolutions to C++ Defect Reports
 - Implemented `CWG2918 Consideration of constraints for address of overloaded `
   `function <https://cplusplus.github.io/CWG/issues/2918.html>`_
 
+- Bumped the ``__cpp_constexpr`` feature-test macro to ``202002L`` in C++20 mode as indicated in
+  `P2493R0 <https://wg21.link/P2493R0>`_.
+
 C Language Changes
 ------------------
 

clang/lib/Frontend/InitPreprocessor.cpp

@@ -664,7 +664,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_lambdas", "200907L");
     Builder.defineMacro("__cpp_constexpr", LangOpts.CPlusPlus26   ? "202406L"
                                            : LangOpts.CPlusPlus23 ? "202211L"
-                                           : LangOpts.CPlusPlus20 ? "201907L"
+                                           : LangOpts.CPlusPlus20 ? "202002L"
                                            : LangOpts.CPlusPlus17 ? "201603L"
                                            : LangOpts.CPlusPlus14 ? "201304L"
                                                                   : "200704");

clang/test/Lexer/cxx-features.cpp

@@ -314,7 +314,7 @@
 #error "wrong value for __cpp_lambdas"
 #endif
 
-#if check(constexpr, 0, 200704, 201304, 201603, 201907, 202211, 202406L)
+#if check(constexpr, 0, 200704, 201304, 201603, 202002, 202211, 202406L)
 #error "wrong value for __cpp_constexpr"
 #endif