We did this for @in => @owned for all parameters before enabling +0. We decided
to defer this work to after +0 was turned back on.
This also fixes the array_contentof_opt test without making append(contentOf: )
take the container at +1.
rdar://38152291
I am going to leave in the infrastructure around this just in case. But there is
no reason to keep this in the tests themselves. I can always just revert this
and I don't think merge conflicts are likely due to previous work I did around
the tooling for this.
Previously ProtocolConformance::subst would crash because it was receiving
things with an unexpected relationship between the conformance's type and the
substituted self type. The compiler doesn't quite properly model "abstract"
inherited conformances, so we end up using normal conformances instead, and we
need to work around this in some cases.
