sanitizers: turn thread-safety errors into warnings

[dvyukov]

Summary

There were 2 issues reported on https://reviews.llvm.org/D105716:

1. FreeBSD phtread.h is annotated with thread-safety attributes

and this causes errors in gtest headers.

2. If sanitizers are compiled with an older versions of clang

(which supports the annotations, but has some false positives
in analysis not present in later versions of clang), compilation
fails with errors.

Switch the errors to warnings by default.
Some CI bots enable COMPILER_RT_ENABLE_WERROR, which should
turn these warnings back into errors.

[nikic]

LGTM, thanks!

compiler-rt/CMakeLists.txt

if(MSVC)
   # initialized, but ASan initializes before the CRT.
   list(APPEND SANITIZER_COMMON_CFLAGS /Zc:threadSafeInit-)
 endif()
 
 append_list_if(COMPILER_RT_DEBUG -DSANITIZER_DEBUG=1 SANITIZER_COMMON_CFLAGS)
 
 if(CMAKE_CXX_COMPILER_ID MATCHES Clang)
   list(APPEND THREAD_SAFETY_FLAGS
-      "-Werror=thread-safety"
-      "-Werror=thread-safety-reference"
-      "-Werror=thread-safety-beta"
+      "-Wthread-safety"
+      "-Wthread-safety-reference"
+      "-Wthread-safety-beta"
   )
   list(APPEND SANITIZER_COMMON_CFLAGS ${THREAD_SAFETY_FLAGS})
   string(REPLACE ";" " " thread_safety_flags_space_sep "${THREAD_SAFETY_FLAGS}")
   string(APPEND COMPILER_RT_TEST_COMPILER_CFLAGS " ${thread_safety_flags_space_sep}")
 endif()
 
 # If we're using MSVC,
 # always respect the optimization flags set by CMAKE_BUILD_TYPE instead.

 set(COMPILER_RT_GTEST_PATH ${LLVM_MAIN_SRC_DIR}/utils/unittest/googletest)
 set(COMPILER_RT_GTEST_SOURCE ${COMPILER_RT_GTEST_PATH}/src/gtest-all.cc)
 set(COMPILER_RT_GTEST_CFLAGS
   -DGTEST_NO_LLVM_SUPPORT=1
   -DGTEST_HAS_RTTI=0
   -I${COMPILER_RT_GTEST_PATH}/include
   -I${COMPILER_RT_GTEST_PATH}
 )
-if(CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
-  # FreeBSD has its pthread functions marked with thread safety annotations, but
-  # googletest is not compatible with such annotations. Disable the thread
-  # safety warnings-as-errors until googletest has been fixed.
-  list(APPEND NO_THREAD_SAFETY_FLAGS ${THREAD_SAFETY_FLAGS})
-  list(TRANSFORM NO_THREAD_SAFETY_FLAGS REPLACE "error=" "no-")
-  list(APPEND COMPILER_RT_GTEST_CFLAGS ${NO_THREAD_SAFETY_FLAGS})
-endif()
 
 # Mocking support.
 set(COMPILER_RT_GMOCK_PATH ${LLVM_MAIN_SRC_DIR}/utils/unittest/googlemock)
 set(COMPILER_RT_GMOCK_SOURCE ${COMPILER_RT_GMOCK_PATH}/src/gmock-all.cc)
 set(COMPILER_RT_GMOCK_CFLAGS
   -DGTEST_NO_LLVM_SUPPORT=1
   -DGTEST_HAS_RTTI=0
   -I${COMPILER_RT_GMOCK_PATH}/include