[sanitizer-coverage] document -fsanitize-coverage=pc-table and...

[sanitizer-coverage] document -fsanitize-coverage=pc-table and -fsanitize-coverage=inline-8bit-counters

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311719 91177308-0d34-0410-b5e6-96231b3b80d8