Prototype support for llvm coverage

https://clang.llvm.org/docs/SourceBasedCodeCoverage.html

Closes #5036.

Usage with statically linked tests (works with clang 3.9 or later):
BAZEL_USE_LLVM_NATIVE_COVERAGE=1 GCOV=llvm-profdata-3.9 CC=clang-3.9 /tmp/bazel coverage --instrumentation_filter=src/main/cpp --dynamic_mode=off src/test/cpp:option_processor_test
(LLVM_COV_CMD="llvm-cov-3.9 show -format=html -output-dir=$(pwd)/report/ -instr-profile bazel-out/k8-fastbuild/testlogs/src/test/cpp/option_processor_test/coverage.dat bazel-out/k8-fastbuild/bin/src/test/cpp/option_processor_test" && cd bazel-os-bazel && $LLVM_COV_CMD)

Usage with dynamically linked tests (requires clang 4.0 or later):
BAZEL_USE_LLVM_NATIVE_COVERAGE=1 GCOV=llvm-profdata-4.0 CC=clang-4.0 bazel coverage --instrumentation_filter=src/main/cpp src/test/cpp:option_processor_test
(LLVM_COV_CMD="llvm-cov-4.0 show -format=html -output-dir=$(pwd)/report/ -instr-profile bazel-out/k8-fastbuild/testlogs/src/test/cpp/option_processor_test/coverage.dat bazel-out/k8-fastbuild/bin/src/test/cpp/option_processor_test $(cat bazel-out/k8-fastbuild/bin/src/test/cpp/option_processor_test.runfiles_manifest | cut -d' ' -f 2 | egrep "\.so$" | xargs -n 1 -I xxx echo -n "-object xxx ")" && cd bazel-os-bazel && $LLVM_COV_CMD)
PiperOrigin-RevId: 194357292