[asan] add interface function __sanitizer_get_total_unique_coverage; useful for coverage-guided in-process fuzzers

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@222060 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master', remote-tracking branch 'trunk/master'