c++: noexcept-spec from nested class confusion [PR109761]

When late processing a noexcept-spec from a nested class after completion
of the outer class (since it's a complete-class context), we pass the wrong
class context to noexcept_override_late_checks -- the outer class type
instead of the nested class type -- which leads to bogus errors in the
below test.

This patch fixes this by making noexcept_override_late_checks obtain the
class context directly via DECL_CONTEXT instead of via an additional
parameter.

	PR c++/109761

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_class_specifier): Don't pass a class
	context to noexcept_override_late_checks.
	(noexcept_override_late_checks): Remove 'type' parameter
	and use DECL_CONTEXT of 'fndecl' instead.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept78.C: New test.

(cherry picked from commit c13906f2)