[cmake] Unmark LLVM_BUILD_INSTRUMENTED_COVERAGE as experimental
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@313769 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 580fbe1a5e29e08c16ebd9c6243a51ec48c32a5f)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm