No longer issue pedantic warning about pre-c++2b compat

We were accidentally issuing "overloaded 'operator[]' with more than
one parameter is a C++2b extension" with -pedantic because it was an
ExtWarn diagnostic rather than a Warning. This corrects the diagnostic
category and adds some test coverage.

Fixes llvm#61582

clang/docs/ReleaseNotes.rst

@@ -1452,6 +1452,10 @@ Bug Fixes to Attribute Support
 
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
+- No longer issue a pre-C++2b compatibility warning in ``-pedantic`` mode
+  regading overloaded `operator[]` with more than one parmeter. (`#61582
+  <https://github.com/llvm/llvm-project/issues/61582>`_)
+
 - Address the thread identification problems in coroutines.
   (`#47177 <https://github.com/llvm/llvm-project/issues/47177>`_,
   `#47179 <https://github.com/llvm/llvm-project/issues/47179>`_)

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9184,7 +9184,7 @@ def err_operator_overload_default_arg : Error<
 
 def ext_subscript_overload : Warning<
   "overloaded %0 with %select{no|a defaulted|more than one}1 parameter is a "
-  "C++23 extension">, InGroup<CXXPre23Compat>, DefaultIgnore;
+  "C++2b extension">, InGroup<CXXPre2bCompat>, DefaultIgnore;
 def error_subscript_overload : Error<
   "overloaded %0 cannot have %select{no|a defaulted|more than one}1 parameter before C++23">;