Add Clang coverage tools support (#30)

[suurjaak]

With regard to issue https://github.com/mikeferguson/code_coverage/issues/30.

To use Clang coverage tools (llvm-cov), user will need to specify:

APPEND_COVERAGE_COMPILER_FLAGS(COMPILER clang)

Existing workflows remain the same as they were, no arguments to function:

APPEND_COVERAGE_COMPILER_FLAGS()

[suurjaak]

@mikeferguson Any comments?

[suurjaak]

@mikeferguson Helloooo?