[PATCH, C++, testsuite] Require alias support for a test.

[Iain Sandoe]

Hi,

The test for pr84497 uses a facility that requires alias support from
the target, and therefore fails on targets without.

Fixed by adding a suitable dg-require statement.

Tested on x86_64-linux-gnu, x86_64-darwin16,
applied to mainline as obvious,
thanks
Iain

2019-05-31  Iain Sandoe  <i...@sandoe.co.uk>

        * g++.dg/cpp0x/pr84497.C: Require alias support.