c++: Add support for vec_dup to constexpr [PR118445]

With the addition of supporting operations on the SVE scalable vector types,
the vec_duplicate tree will show up in expressions and the constexpr handling
was not done for this tree code.
This is a simple fix to treat VEC_DUPLICATE like any other unary operator and allows
the constexpr-add-1.C testcase to work.

Built and tested for aarch64-linux-gnu.

	PR c++/118445

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Handle VEC_DUPLICATE like
	a "normal" unary operator.
	(potential_constant_expression_1): Likewise.

gcc/testsuite/ChangeLog:

	* g++.target/aarch64/sve/constexpr-add-1.C: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>