Add -lrt to sanitizer link arguments.

Sanitizer runtime intercepts functions from librt. Not doing this will fail
if the librt dependency is not present at program startup (ex. comes from a
dlopen()ed library).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182645 91177308-0d34-0410-b5e6-96231b3b80d8