c++: ICE with unparsed noexcept [PR117106]

In a member-specification of a class, a noexcept-specifier is
a complete-class context.  Thus we delay parsing until the end of
the class via our DEFERRED_PARSE mechanism; see cp_parser_save_noexcept
and cp_parser_late_noexcept_specifier.

We also attempt to defer instantiation of noexcept-specifiers in order
to reduce the number of instantiations; this is done via DEFERRED_NOEXCEPT.

We can even have both, as in noexcept65.C: a DEFERRED_PARSE wrapped in
DEFERRED_NOEXCEPT, which uses the DEFPARSE_INSTANTIATIONS mechanism.
noexcept65.C works, because when we really need the noexcept, which is
when parsing the body of S::A::A(), the noexcept will have been parsed
already; noexcepts are parsed before bodies of member function.

But in this test we have:

  struct A {
      int x;
      template<class>
      void foo() noexcept(noexcept(x)) {}
      auto bar() -> decltype(foo<int>()) {} // #1
  };

and I think the decltype in #1 needs the unparsed noexcept before it
could have been parsed.  clang++ rejects the test and I suppose we
should reject it as well, rather than crashing on a DEFERRED_PARSE
in tsubst_expr.

	PR c++/117106
	PR c++/118190

gcc/cp/ChangeLog:

	* pt.cc (maybe_instantiate_noexcept): Give an error if the noexcept
	hasn't been parsed yet.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept89.C: New test.
	* g++.dg/cpp0x/noexcept90.C: New test.

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

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