C++ PATCH for c++/90881 - bogus -Wunused-value in unevaluated context

[Jason Merrill]

On 6/13/19 9:21 PM, Marek Polacek wrote:
> As Jon points out in the PR, emitting -Wunused warnings in unevaluated contexts
> doesn't make a whole lot of sense, because in such contexts, we're not reading
> the values in any case.  Disabling this particular warning is trivial.  There
> are likely other warnings like this, but I haven't audited them.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?

OK.

Jason