hwc: enable code coverage if CLANG_COVERAGE=true

This will enable CLANG's source-based code coverage with continuous mode
enabled (profile counter updates are continuously synced to a file).

To set the output directory, we need to define environment variable
LLVM_PROFILE_FILE to like /data/misc/trace/clang-hwc-%20m%c.profraw
in HWC's init (.rc) file.

Bug: 295827160
Test: builds
Change-Id: I88fb597a85819a23491143e716cb08cc6152ff6a
3 files changed