c++: don't default -frange-for-ext-temps in -std=gnu++20 [PR188574]

c++: don't default -frange-for-ext-temps in -std=gnu++20 [PR188574]

Since -frange-for-ext-temps has been causing trouble, let's not enable it
by default in pre-C++23 GNU modes for GCC 15, and also allow disabling it in
C++23 and up.

	PR c++/188574

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): Only enable
	-frange-for-ext-temps by default in C++23.

gcc/ChangeLog:

	* doc/invoke.texi: Adjust -frange-for-ext-temps documentation.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/range-for3.C: Use -frange-for-ext-temps.
	* g++.dg/cpp23/range-for4.C: Adjust expected result.

libgomp/ChangeLog:

	* testsuite/libgomp.c++/range-for-4.C: Adjust expected result.