[cmake] By default do not instrument compiler-rt if LLVM_BUILD_INSTRUMENTED_COVERAGE is ON

Applying the same rules as for LLVM_BUILD_INSTRUMENTED build in the cmake files.
By having this patch, we are able to disable/enable instrument+coverage build
of the compiler-rt project when building instrumented LLVM.

Differential Revision: https://reviews.llvm.org/D108127