Add acov-llvm.py script

Bug: http://b/160401633

This is a tool for gathering coverage information from a device and
generating an LLVM coverage report from that information.  See top of
the file for usage instructions.

Also redirect acov users to acov-llvm when CLANG_COVERAGE is set.

Test: acov-llvm.py wih clean-device, flush and report subcommands.

Change-Id: I5dba3a669bb6ac2797093903536f346228625c05
2 files changed