c++: Fix noexcept checking for trivial operations [PR96090]

This patch stops eager folding of trivial operations (construction and
assignment) from occurring when checking for noexceptness. This was
previously done in PR c++/53025, but only for copy/move construction,
and the __is_nothrow_xible builtins did not receive the same treatment
when they were added.

To handle `is_nothrow_default_constructible`, the patch also ensures
that when no parameters are passed we do value initialisation instead of
just building the constructor call: in particular, value-initialisation
doesn't necessarily actually invoke the constructor for trivial default
constructors, and so we need to handle this case as well.

This is contrary to the proposed resolution of CWG2820; for now we just
ensure it matches the behaviour of the `noexcept` operator and create
testcases formalising this, and if that issue gets accepted we can
revisit.

	PR c++/96090
	PR c++/100470

gcc/cp/ChangeLog:

	* call.cc (build_over_call): Prevent folding of trivial special
	members when checking for noexcept.
	* method.cc (constructible_expr): Perform value-initialisation
	for empty parameter lists.
	(is_nothrow_xible): Treat as noexcept operator.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept81.C: New test.
	* g++.dg/ext/is_nothrow_constructible7.C: New test.
	* g++.dg/ext/is_nothrow_constructible8.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

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