[libunwind] Don't remove defaultlibs

[philnik]

Summary

Clang uses -nostdlib++ instead of -nodefaultlibs, so the only problematic libraries are C++ libraries. We already invoke the C frontend instead of the C++ one, so removing and re-adding all the defaultlibs is a lot of work without any benefit.

[philnik]

Other fix

libunwind/cmake/config-ix.cmake

 # otherwise -nodefaultlibs is used. We want all our checks to also
 # use one of these options, otherwise we may end up with an inconsistency between
 # the flags we think we require during configuration (if the checks are
 # performed without one of those options) and the flags that are actually
 # required during compilation (which has the -nostdlib++ or -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
 
-llvm_check_compiler_linker_flag(CXX "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
-else()
-  llvm_check_compiler_linker_flag(C "-nodefaultlibs" C_SUPPORTS_NODEFAULTLIBS_FLAG)
-  if (C_SUPPORTS_NODEFAULTLIBS_FLAG)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
-  endif()
-endif()
 
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
+if (C_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBUNWIND_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBUNWIND_USE_COMPILER_RT)
     include(HandleCompilerRT)
     find_compiler_rt_library(builtins LIBUNWIND_BUILTINS_LIBRARY
                              FLAGS ${LIBUNWIND_COMPILE_FLAGS})
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBUNWIND_BUILTINS_LIBRARY}")

libunwind/src/CMakeLists.txt

 add_library_flags_if(LIBUNWIND_HAS_DL_LIB dl)
 if (LIBUNWIND_ENABLE_THREADS)
   add_library_flags_if(LIBUNWIND_HAS_PTHREAD_LIB pthread)
   add_compile_flags_if(LIBUNWIND_WEAK_PTHREAD_LIB -DLIBUNWIND_USE_WEAK_PTHREAD=1)
 endif()
 
 # Setup flags.
 add_link_flags_if(CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG --unwindlib=none)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-  add_link_flags_if_supported(-nostdlib++)
-else()
-  add_link_flags_if_supported(-nodefaultlibs)
-endif()
+
 
 # MINGW_LIBRARIES is defined in config-ix.cmake
 add_library_flags_if(MINGW "${MINGW_LIBRARIES}")
 
 if (LIBUNWIND_ENABLE_SHARED AND
     NOT (CXX_SUPPORTS_FNO_EXCEPTIONS_FLAG AND
          CXX_SUPPORTS_FUNWIND_TABLES_FLAG))
   message(FATAL_ERROR