Revert "[sancov] moving sancov rt to sancov/ directory"

This reverts commit https://reviews.llvm.org/rL291734
Reason: mac breakage
http://lab.llvm.org:8080/green//job/clang-stage1-configure-RA_build/28798/consoleFull#1657087648e9a0fee5-ebcc-4238-a641-c5aa112c323e

llvm-svn: 291736