[libFuzzer] Update LibFuzzer w.r.t. the new comparisons instrumentation API

Added the _sanitizer_cov_trace_const_cmp[1248] callbacks.
For now they are implemented the same way as _sanitizer_cov_trace_cmp[1248].
For more details, please see https://reviews.llvm.org/D36465.

Patch by Victor Chibotaru.

llvm-svn: 310592