[cmake] Unmark LLVM_BUILD_INSTRUMENTED_COVERAGE as experimental

The coverage bot has been stable for a while:

  http://lab.llvm.org:8080/coverage/coverage-reports/index.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313769 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed