libstdc++: Break header cycle between <new> and <exception>

libstdc++: Break header cycle between <new> and <exception>

The <new> and <exception> headers each include each other, which makes
building them as header-units "exciting". The <new> header only needs
the definition of std::exception (in order to derive from it) which is
already in its own header, so just include that.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h: Include <bits/exception_defines.h>
	for definitions of __try, __catch and __throw_exception_again.
	(counted_iterator::operator++(int)): Use __throw_exception_again
	instead of throw.
	* libsupc++/new: Include <bits/exception.h> not <exception>.
	* libsupc++/new_opvnt.cc: Include <bits/exception_defines.h>.
	* testsuite/18_support/destroying_delete.cc: Include
	<type_traits> for std::is_same_v definition.
	* testsuite/20_util/variant/index_type.cc: Qualify size_t.