[Clang] Set __cpp_explicit_this_parameter (#107451)

There are not a lot of outstanding known issues
with deducing this (besides #95112), so it
seems reasonable to claim full support.

Fixes #82780

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -272,6 +272,8 @@ C++23 Feature Support
 
 - Extend lifetime of temporaries in mem-default-init for P2718R0. Clang now fully
   supports `P2718R0 Lifetime extension in range-based for loops <https://wg21.link/P2718R0>`_.
+  
+- ``__cpp_explicit_this_parameter`` is now defined. (#GH82780)
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/lib/Frontend/InitPreprocessor.cpp

@@ -752,6 +752,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_if_consteval", "202106L");
     Builder.defineMacro("__cpp_multidimensional_subscript", "202211L");
     Builder.defineMacro("__cpp_auto_cast", "202110L");
+    Builder.defineMacro("__cpp_explicit_this_parameter", "202110L");
   }
 
   // We provide those C++23 features as extensions in earlier language modes, so

clang/test/Lexer/cxx-features.cpp

@@ -81,7 +81,7 @@
 #error "wrong value for __cpp_named_character_escapes"
 #endif
 
-#if check(explicit_this_parameter, 0, 0, 0, 0, 0, 0, 0)
+#if check(explicit_this_parameter, 0, 0, 0, 0, 0, 202110L, 202110L)
 #error "wrong value for __cpp_explicit_this_parameter"
 #endif