[libc++][NFC] Add reminder to remove _LIBCPP_HAS_EXPLICIT_THIS_PARAMETER macro (#127070)

`__cpp_explicit_this_parameter` has been set in LLVM20
llvm/llvm-project#107451 /
llvm/llvm-project@72e58e0

---------

Co-authored-by: Hristo Hristov <[email protected]>

Author: H-G-Hristov

libcxx/include/__config

@@ -1198,6 +1198,7 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 #  endif
 
+// TODO(LLVM 22): Remove this macro once LLVM19 support ends. __cpp_explicit_this_parameter has been set in LLVM20.
 // Clang-18 has support for deducing this, but it does not set the FTM.
 #  if defined(__cpp_explicit_this_parameter) || (defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER >= 1800)
 #    define _LIBCPP_HAS_EXPLICIT_THIS_PARAMETER 1