c++: deleted fn and noexcept inst [PR101532, PR104225]

c++: deleted fn and noexcept inst [PR101532, PR104225]

Here when attempting to use B's implicitly deleted default constructor,
mark_used rightfully returns false, but for the wrong reason: it
tries to instantiate the synthesized noexcept specifier which then only
silently fails because get_defaulted_eh_spec suppresses diagnostics
for deleted functions.  This lack of diagnostics causes us to crash on
the first testcase below (thanks to the assert in finish_expr_stmt), and
silently accept the second testcase.

To fix this, this patch makes mark_used avoid attempting to instantiate
the noexcept specifier of a deleted function, so that we'll instead
directly reject (and diagnose) the function due to its deletedness.

	PR c++/101532
	PR c++/104225

gcc/cp/ChangeLog:

	* decl2.c (mark_used): Don't consider maybe_instantiate_noexcept
	on a deleted function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/nsdmi-template21.C: New test.
	* g++.dg/cpp0x/nsdmi-template21a.C: New test.

(cherry picked from commit bc90dd0ecf02e11d47d1af7f627e2e2acaa40106)