Add cmake flag LLVM_USE_SANITIZE_COVERAGE

[kcc]

Summary

When LLVM_USE_SANITIZE_COVERAGE=YES
and one of the sanitizers is used -fsanitize-coverage=3 will be added
to build flag. This will be used to run a coverage-guided fuzzer on various
llvm libraries.

[rnk]

lgtm

cmake/modules/HandleLLVMOptions.cmake

elseif (LLVM_USE_SANITIZER STREQUAL "Thread")
       append_common_sanitizer_flags()
       append("-fsanitize=thread" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     else()
       message(WARNING "Unsupported value of LLVM_USE_SANITIZER: ${LLVM_USE_SANITIZER}")
     endif()
   else()
     message(WARNING "LLVM_USE_SANITIZER is not supported on this platform.")
   endif()
+  if (LLVM_USE_SANITIZE_COVERAGE)
+    append("-fsanitize-coverage=3" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  endif()
 endif()
 
 # Turn on -gsplit-dwarf if requested
 if(LLVM_USE_SPLIT_DWARF)
   add_definitions("-gsplit-dwarf")
 endif()
 
 add_llvm_definitions( -D__STDC_CONSTANT_MACROS )