[v3] constexpr array additions

[Benjamin Kosnik]

>      I think the -fno-exceptions should be removed from the test 
> dg-options. Otherwise your comment saying that the 'at' should throw
> or abort is wrong. I think the testsuite logic is to write neutral
> tests and challenge them with numerous different combination of
> options. I guess the testsuite is run with -fno-exceptions option
> before a release, no ?

Yeah. You are right. Will fix: thanks for the comments.

>      Moreover you are testing the mutable at method, not the const
> one involved in the patch, we should have a second test case.

Ouch. Thanks for catching this.

-benjamin