[gn] port 8a7846f (C++23 for libcxx, libcxxabi)

Author: nico

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

@@ -49,7 +49,7 @@ config("cxx_config") {
     "-Wno-covered-switch-default",
   ]
   cflags_cc = [
-    "-std=c++20",
+    "-std=c++23",
     "-nostdinc++",
   ]
   defines = [

llvm/utils/gn/secondary/libcxxabi/src/BUILD.gn

@@ -66,7 +66,7 @@ config("cxxabi_config") {
     "//libcxx/src",
   ]
   cflags_cc = [
-    "-std=c++20",
+    "-std=c++23",
     "-nostdinc++",
   ]
   defines = [