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

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

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