[OpenMP] Silence more warning flags

[mstorsjo]

Summary

This silences warnings like these, in mingw builds with clang:

runtime/src/kmp_atomic.h:1021:13: warning: '__kmpc_atomic_cmplx8_rd' has C-linkage specified, but returns user-defined type 'kmp_cmplx64' (aka '__kmp_cmplx64_t') which is incompatible with C [-Wreturn-type-c-linkage]

runtime/src/z_Windows_NT_util.cpp:479:17: warning: cast from 'volatile void *' to 'type-parameter-0-0 *' drops volatile qualifier [-Wcast-qual]
    flag = (C *)th->th.th_sleep_loc;

runtime/src/z_Windows_NT_util.cpp:1321:14: warning: cast to 'void *' from smaller integer type 'DWORD' (aka 'unsigned long') [-Wint-to-void-pointer-cast]
  } else if ((void *)exit_val != (void *)th) {

[AndreyChurbanov]

LGTM

openmp/runtime/cmake/LibompHandleFlags.cmake

function(libomp_get_cxxflags cxxflags)
   libomp_append(flags_local -Wno-covered-switch-default LIBOMP_HAVE_WNO_COVERED_SWITCH_DEFAULT_FLAG)
   libomp_append(flags_local -Wno-frame-address LIBOMP_HAVE_WNO_FRAME_ADDRESS_FLAG)
   libomp_append(flags_local -Wno-strict-aliasing LIBOMP_HAVE_WNO_STRICT_ALIASING_FLAG)
   libomp_append(flags_local -Wstringop-overflow=0 LIBOMP_HAVE_WSTRINGOP_OVERFLOW_FLAG)
   libomp_append(flags_local -Wno-stringop-truncation LIBOMP_HAVE_WNO_STRINGOP_TRUNCATION_FLAG)
   libomp_append(flags_local -Wno-switch LIBOMP_HAVE_WNO_SWITCH_FLAG)
   libomp_append(flags_local -Wno-uninitialized LIBOMP_HAVE_WNO_UNINITIALIZED_FLAG)
   libomp_append(flags_local -Wno-unused-but-set-variable LIBOMP_HAVE_WNO_UNUSED_BUT_SET_VARIABLE_FLAG)
+  libomp_append(flags_local -Wno-return-type-c-linkage LIBOMP_HAVE_WNO_RETURN_TYPE_C_LINKAGE_FLAG)
+  libomp_append(flags_local -Wno-cast-qual LIBOMP_HAVE_WNO_CAST_QUAL_FLAG)
+  libomp_append(flags_local -Wno-int-to-void-pointer-cast LIBOMP_HAVE_WNO_INT_TO_VOID_POINTER_CAST_FLAG)
   # libomp_append(flags_local -Wconversion LIBOMP_HAVE_WCONVERSION_FLAG)
   libomp_append(flags_local /GS LIBOMP_HAVE_GS_FLAG)
   libomp_append(flags_local /EHsc LIBOMP_HAVE_EHSC_FLAG)
   libomp_append(flags_local /Oy- LIBOMP_HAVE_OY__FLAG)
   libomp_append(flags_local -mrtm LIBOMP_HAVE_MRTM_FLAG)
   # Intel(R) C Compiler flags
   libomp_append(flags_local /Qsafeseh LIBOMP_HAVE_QSAFESEH_FLAG)
   libomp_append(flags_local -Qoption,cpp,--extended_float_types LIBOMP_HAVE_EXTENDED_FLOAT_TYPES_FLAG)

openmp/runtime/cmake/config-ix.cmake

 check_cxx_compiler_flag(-Wno-covered-switch-default LIBOMP_HAVE_WNO_COVERED_SWITCH_DEFAULT_FLAG)
 check_cxx_compiler_flag(-Wno-frame-address LIBOMP_HAVE_WNO_FRAME_ADDRESS_FLAG)
 check_cxx_compiler_flag(-Wno-strict-aliasing LIBOMP_HAVE_WNO_STRICT_ALIASING_FLAG)
 check_cxx_compiler_flag(-Wstringop-overflow=0 LIBOMP_HAVE_WSTRINGOP_OVERFLOW_FLAG)
 check_cxx_compiler_flag(-Wno-stringop-truncation LIBOMP_HAVE_WNO_STRINGOP_TRUNCATION_FLAG)
 check_cxx_compiler_flag(-Wno-switch LIBOMP_HAVE_WNO_SWITCH_FLAG)
 check_cxx_compiler_flag(-Wno-uninitialized LIBOMP_HAVE_WNO_UNINITIALIZED_FLAG)
 check_cxx_compiler_flag(-Wno-unused-but-set-variable LIBOMP_HAVE_WNO_UNUSED_BUT_SET_VARIABLE_FLAG)
+check_cxx_compiler_flag(-Wno-return-type-c-linkage LIBOMP_HAVE_WNO_RETURN_TYPE_C_LINKAGE_FLAG)
+check_cxx_compiler_flag(-Wno-cast-qual LIBOMP_HAVE_WNO_CAST_QUAL_FLAG)
+check_cxx_compiler_flag(-Wno-int-to-void-pointer-cast LIBOMP_HAVE_WNO_INT_TO_VOID_POINTER_CAST_FLAG)
 # check_cxx_compiler_flag(-Wconversion LIBOMP_HAVE_WCONVERSION_FLAG)
 check_cxx_compiler_flag(-msse2 LIBOMP_HAVE_MSSE2_FLAG)
 check_cxx_compiler_flag(-ftls-model=initial-exec LIBOMP_HAVE_FTLS_MODEL_FLAG)
 libomp_check_architecture_flag(-mmic LIBOMP_HAVE_MMIC_FLAG)
 libomp_check_architecture_flag(-m32 LIBOMP_HAVE_M32_FLAG)
 if(WIN32)
   if(MSVC)
     # Check Windows MSVC style flags.