[OpenACC] Fix invalid using inside of an openacc directive (#198058)
Bug report #197858 comes up with a reproducer where an invalid `using`
declaration checks the Scope it is in, and asserts if it isn't in a
DeclScope. Since all of the important directives that create scopes end
up causing a new scope anyway, this patch adds 'DeclScope' to the parse
scope for an OpenACC directive. This follows the guidance of the OpenMP
directives.
Fixes: #197858