Update __cpp_concepts macro (#87998)

After discussion with a few others, and seeing the state of our concepts
support, I believe it is worth trying to see if we can update this for
Clang19. The forcing function is that libstdc++'s `<expected>` header is
guarded by this macro, so we need to update it to support that.

Author: erichkeane

clang/docs/ReleaseNotes.rst

@@ -110,6 +110,10 @@ C++20 Feature Support
   templates (`P1814R0 <https://wg21.link/p1814r0>`_).
   (#GH54051).
 
+- We have sufficient confidence and experience with the concepts implementation
+  to update the ``__cpp_concepts`` macro to `202002L`. This enables
+  ``<expected>`` from libstdc++ to work correctly with Clang.
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Frontend/InitPreprocessor.cpp

@@ -720,10 +720,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   if (LangOpts.CPlusPlus20) {
     Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
 
-    // P0848 is implemented, but we're still waiting for other concepts
-    // issues to be addressed before bumping __cpp_concepts up to 202002L.
-    // Refer to the discussion of this at https://reviews.llvm.org/D128619.
-    Builder.defineMacro("__cpp_concepts", "201907L");
+    Builder.defineMacro("__cpp_concepts", "202002");
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
     Builder.defineMacro("__cpp_consteval", "202211L");
     Builder.defineMacro("__cpp_constexpr_dynamic_alloc", "201907L");

clang/test/Lexer/cxx-features.cpp

@@ -85,7 +85,7 @@
 #error "wrong value for __cpp_char8_t"
 #endif
 
-#if check(concepts, 0, 0, 0, 0, 201907, 201907, 201907)
+#if check(concepts, 0, 0, 0, 0, 202002, 202002, 202002)
 #error "wrong value for __cpp_concepts"
 #endif
 

clang/www/cxx_status.html

@@ -544,16 +544,8 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr> <!-- from Cologne -->
         <td><a href="https://wg21.link/p0848r3">P0848R3</a></td>
-        <td rowspan="1" class="partial" align="center">
-          <details>
-            <summary>Clang 16 (Partial)</summary>
-            Because of other concepts implementation deficits, the __cpp_concepts macro is not yet set to 202002L.
-            Also, the related defect reports <a href="https://wg21.link/cwg1496">DR1496</a> and
-            <a href="https://wg21.link/cwg1734">DR1734</a> are not yet implemented. Accordingly, deleted
-            special member functions are treated as eligible even though they shouldn't be.
-          </details>
-        </td>
-        </tr>
+        <td rowspan="1" class="full" align="center">Clang 19</td>
+      </tr>
       <tr>
         <td><a href="https://wg21.link/p1616r1">P1616R1</a></td>
         <td rowspan="2" class="full" align="center">Clang 10</td>