[scudo] Use -fsanitize=scudo rather than --whole-archive in tests

Summary:
Tests were being run by whole-linking the static library with our test binaries.
But since `-fsanitize=scudo` landed with rL317337, we might as well change how
the tests are compiled to use it.

The only difference will be on Android, where the clang flag links in the
dynamic library instead, but the bots are already pushing
`libclang_rt.*-android.so` to the device there is no additional change needed.

Tested locally, including with a standalone build, and an Android one on a O
device, and it all passes.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42243

llvm-svn: 322882