c++: cxx_eval_array_reference and empty elem type [PR101194]

c++: cxx_eval_array_reference and empty elem type [PR101194]

Here the initializer for x is represented as an empty CONSTRUCTOR due to
its empty element type.  So during constexpr evaluation of the ARRAY_REF
x[0], we end up trying to value initialize the omitted element at index 0,
which fails because the element type is not default constructible.

This patch makes cxx_eval_array_reference specifically handle the case
where the element type is an empty type.

	PR c++/101194

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_array_reference): When the element type
	is an empty type and the corresponding element is omitted, just
	return an empty CONSTRUCTOR instead of attempting value
	initialization.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty16.C: New test.

(cherry picked from commit a688c284dd3848b6c4ea553035f0f9769fb4fbc9)