[Clang] Set the FTM for trivial relocation (#142936)

The language of side seems fairly stable.
Setting the feature test macro will ease implementation in standard
libraries.

Author: cor3ntin

clang/lib/Frontend/InitPreprocessor.cpp

@@ -766,7 +766,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   Builder.defineMacro("__cpp_pack_indexing", "202311L");
   Builder.defineMacro("__cpp_deleted_function", "202403L");
   Builder.defineMacro("__cpp_variadic_friend", "202403L");
-  // Builder.defineMacro("__cpp_trivial_relocatability", "202502L");
+  Builder.defineMacro("__cpp_trivial_relocatability", "202502L");
 
   if (LangOpts.Char8)
     Builder.defineMacro("__cpp_char8_t", "202207L");

clang/test/Lexer/cxx-features.cpp

@@ -49,6 +49,10 @@
 #error "wrong value for __cpp_placeholder_variables"
 #endif
 
+#if check(trivial_relocatability, 202502, 202502, 202502, 202502, 202502, 202502, 202502)
+#error "wrong value for __cpp_trivial_relocatability"
+#endif
+
 // --- C++23 features ---
 
 #if check(auto_cast, 0, 0, 0, 0, 0, 202110, 202110)

clang/www/cxx_status.html

@@ -280,12 +280,7 @@ <h2 id="cxx26">C++2c implementation status</h2>
  <tr>
   <td>Trivial Relocatability</pre></td>
   <td><a href="https://wg21.link/P2786">P2786R13</a></td>
-  <td class="partial" align="center">
-    <details>
-      <summary>Clang 21 (Partial)</summary>
-      The feature test macro (<code>__cpp_trivial_relocatability</code>) has not yet been set.
-    </details>
-  </td>
+  <td class="unreleased" align="center">Clang 21</td>
  </tr>
  <tr>
   <td><pre>#embed</pre></td>