Always compile libFuzzer with no coverage

[george.karpenkov]

Summary

Do not compile libFuzzer itself with coverage, regardless of LLVM variables

[kcc]

-fsanitize-coverage=0 maybe?

[george.karpenkov]

Updated to make GCC-compatible.
@kcc: is that sufficient? The simplification is orthogonal to this patch though.

[kcc]

-fsanitize-coverage=0 should be sufficient.
An explicit list may not be sufficient

[kcc]

LGTM

lib/Fuzzer/CMakeLists.txt

else()
   set(LIBFUZZER_ENABLED_CHECK OFF)
 endif()
 
 # Compile libFuzzer if the compilation is specifically requested, OR
 # if the platform is known to be working.
 set(LIBFUZZER_ENABLE ${LIBFUZZER_ENABLED_CHECK} CACHE BOOL "Build libFuzzer and its tests")
 set(LIBFUZZER_ENABLE_TESTS OFF CACHE BOOL "Build libFuzzer and its tests")
 
-if (LLVM_USE_SANITIZE_COVERAGE)
+if (CMAKE_CXX_FLAGS MATCHES "fsanitize-coverage")
   set(CMAKE_CXX_FLAGS
-    "${CMAKE_CXX_FLAGS} -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters -Werror")
+    "${CMAKE_CXX_FLAGS} -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters")
 endif()
 
 if (LIBFUZZER_ENABLE)
   add_library(LLVMFuzzerNoMainObjects OBJECT
       FuzzerCrossOver.cpp
       FuzzerDriver.cpp
       FuzzerExtFunctionsDlsym.cpp
       FuzzerExtFunctionsDlsymWin.cpp