[Sanitizers] Don't use clang_rt.sancov_{begin,end} on Solaris

As explained in https://reviews.llvm.org/D63601, there's no point using clang_rt.sancov_{begin,end}
on Solaris any longer.

This companion patch to the above removes their use from the driver.

Tested on amd64-pc-solaris2.11

Differential Revision: https://reviews.llvm.org/D63602

llvm-svn: 365396