Revert "Make -fsanitize=scudo use scudo_standalone. Delete check-scudo."

It broke the build, see comments on code review.

> Leaves the implementation and tests files in-place for right now, but
> deletes the ability to build the old sanitizer-common based scudo. This
> has been on life-support for a long time, and the newer scudo_standalone
> is much better supported and maintained.
>
> Also patches up some GWP-ASan wording, primarily related to the fact
> that -fsanitize=scudo now is scudo_standalone, and therefore the way to
> reference the GWP-ASan options through the environment variable has
> changed.
>
> Future follow-up patches will delete the original scudo, and migrate all
> its tests over to be part of the scudo_standalone test suite.
>
> Reviewed By: vitalybuka
>
> Differential Revision: https://reviews.llvm.org/D138157

This reverts commit ab1a5991.