add coverage compile option

[littleDrew]

add coverage compile option for better code testing rate detect

[derekmauro]

In general our policy is to limit the number of build flags. Each build flag creates a combinatoric explosion of possible build modes to test and maintain. In this specific case, you shouldn't need a flag at all. You should just be able to do something like like:

cmake -DABSL_BUILD_TESTING=ON -DABSL_USE_GOOGLETEST_HEAD=ON -DCMAKE_CXX_FLAGS="-O0 -fprofile-arcs -ftest-coverage" ..

In regards to Abseil's test coverage, thanks for reminding me that we still have some tests that we run internally that we haven't open-sourced.