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
diff --git a/scripts/acov b/scripts/acov
index 3524280..0ad9836 100755
--- a/scripts/acov
+++ b/scripts/acov
@@ -33,6 +33,11 @@
 # it in your browser).
 #
 
+if [ "$CLANG_COVERAGE" == "true" ]; then
+  echo "CLANG_COVERAGE is set.  Use development/scripts/acov-llvm.py instead."
+  exit 0
+fi
+
 ANDROID_OUT=${OUT_DIR:-out}
 FLUSH_SLEEP=${FLUSH_SLEEP:-60}