Fix libc++ configuration with -fsanitize-coverage

[krasin]

Summary

a recent change (r280015) in libc++ configuration broke LibFuzzer bot:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer/builds/12245

It's not restricted just to that bot; any code that uses the sanitize coverage and configures libc++ hits it.

This CL fixes the issue.

cmake/Modules/CheckLibcxxAtomic.cmake

function(check_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_FLAGS "-nodefaultlibs -std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
   if (${LIBCXX_GCC_TOOLCHAIN})
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
   endif()
   if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
   endif()
+  if (CMAKE_C_FLAGS MATCHES -fsanitize-coverage OR CMAKE_CXX_FLAGS MATCHES -fsanitize-coverage)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")
+  endif()
   check_cxx_source_compiles("
 #include <cstdint>
 #include <atomic>
 std::atomic<uintptr_t> x;
 std::atomic<uintmax_t> y;
 int main() {
   return x + y;
 }

cmake/config-ix.cmake

if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBCXX_USE_COMPILER_RT)
     list(APPEND CMAKE_REQUIRED_LIBRARIES -rtlib=compiler-rt)
   elseif (LIBCXX_HAS_GCC_S_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
   endif ()
   if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
   endif ()
+  if (CMAKE_C_FLAGS MATCHES -fsanitize-coverage OR CMAKE_CXX_FLAGS MATCHES -fsanitize-coverage)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")
+  endif ()
 endif ()
 
 include(CheckLibcxxAtomic)
 
 # Check compiler flags
 
 check_cxx_compiler_flag(/WX                     LIBCXX_HAS_WX_FLAG)
 check_cxx_compiler_flag(/WX-                    LIBCXX_HAS_NO_WX_FLAG)