c++: canonicity of fn types w/ instantiated eh specs [PR115223]

c++: canonicity of fn types w/ instantiated eh specs [PR115223]

When propagating structural equality in build_cp_fntype_variant, we
should consider structural equality of the exception-less variant, not
of the given type which might use structural equality only because it
has a (complex) noexcept-spec that we're intending to replace, as in
maybe_instantiate_noexcept which calls build_exception_variant using
the deferred-noexcept function type.  Otherwise we might pessimistically
use structural equality for a function type with a simple instantiated
noexcept-spec, leading to a LTO-triggered type verification failure if we
later use that (structural-equality) type as the canonical version of
some other variant.

	PR c++/115223

gcc/cp/ChangeLog:

	* tree.cc (build_cp_fntype_variant): Propagate structural
	equality of the exception-less variant.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept87.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>