[sancov] specifying sanitizer coverage dependencies.

Summary:
These dependencies would be used in the future to reduce the number
of instrumented blocks(http://reviews.llvm.org/rL262103)

This is submitted as a separate CL because of previous problems with
ARM.

Subscribers: aemerson

Differential Revision: http://reviews.llvm.org/D18227

llvm-svn: 263797