c++ testsuite: Add test for PR107703

c++ testsuite: Add test for PR107703

This is on top of the
https://gcc.gnu.org/pipermail/gcc-patches/2022-November/606398.html
and
https://gcc.gnu.org/pipermail/gcc-patches/2023-March/613724.html
patches (to be precise, the latter isn't essential for it), I've
realized that for the PR107703 bugfix in the first patch I haven't
added some test coverage that the extended floating vs. integral
or vice versa conversions work correctly.

This new testcase adds such checks.  And when writing it I've
found that in ext-floating.h header in the testsuite I forgot back
in November to remove #undef __STDCPP_BFLOAT16_T__ which was left
there because the bfloat16 support wasn't in yet.

The new testcase (and all older ext-floating*.C tests too) passes
on vanilla trunk without the ext-floating.h change (x86_64-linux
-m32/-m64) and with the PR107703 fix also with the ext-floating.h
change.

2023-03-10  Jakub Jelinek  <jakub@redhat.com>

	PR target/107703
	* g++.dg/cpp23/ext-floating.h (__STDCPP_BFLOAT16_T__): Don't undefine
	it.
	(std::bfloat16_t): Use decltype (0.0bf16) like libstdc++, rather than
	__bf16.
	* g++.dg/cpp23/ext-floating14.C: New test.