[CMake][AIX] Disable visibility options in build

[daltenty]

Summary

Visibility options currently have limited support on AIX and may cause warnings or errors
depending on the build compiler used.

[ZarkoCA]

This LGTM.

llvm/cmake/modules/HandleLLVMOptions.cmake

if( LLVM_ENABLE_PIC )
   # clang with -O3 -fPIC generates code that SEGVs.
   # Both can be worked around by compiling with -O instead.
   if(${CMAKE_SYSTEM_NAME} STREQUAL "SunOS" AND LLVM_NATIVE_ARCH STREQUAL "Sparc")
     llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O[23]" "-O")
     llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O[23]" "-O")
   endif()
 endif()
 
-if(NOT WIN32 AND NOT CYGWIN AND NOT (${CMAKE_SYSTEM_NAME} MATCHES "AIX" AND CMAKE_CXX_COMPILER_ID STREQUAL "GNU"))
+if(NOT WIN32 AND NOT CYGWIN AND NOT (${CMAKE_SYSTEM_NAME} MATCHES "AIX"))
   # MinGW warns if -fvisibility-inlines-hidden is used.
-  # GCC on AIX warns if -fvisibility-inlines-hidden is used.
+  # GCC on AIX warns if -fvisibility-inlines-hidden is used and Clang on AIX doesn't currently support visibility.
   check_cxx_compiler_flag("-fvisibility-inlines-hidden" SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG)
   append_if(SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG "-fvisibility-inlines-hidden" CMAKE_CXX_FLAGS)
 endif()
 
 if(CMAKE_SIZEOF_VOID_P EQUAL 8 AND MINGW)
   add_definitions( -D_FILE_OFFSET_BITS=64 )
 endif()
 

llvm/lib/Target/CMakeLists.txt

add_llvm_component_library(LLVMTarget
   Core
   MC
   Support
   )
 
 # When building shared objects for each target there are some internal APIs
 # that are used across shared objects which we can't hide.
 if (NOT BUILD_SHARED_LIBS AND NOT APPLE AND
+    NOT (${CMAKE_SYSTEM_NAME} MATCHES "AIX") AND
     NOT DEFINED CMAKE_CXX_VISIBILITY_PRESET)
   # Set default visibility to hidden, so we don't export all the Target classes
   # in libLLVM.so.
   set(CMAKE_CXX_VISIBILITY_PRESET hidden)
 endif()
 
 foreach(t ${LLVM_TARGETS_TO_BUILD})
   message(STATUS "Targeting ${t}")
   add_subdirectory(${t})
 endforeach()
 
 # Currently we do not allow libraries from lib to reference targets directly.
 # This property is used to enforce that convention. It is important because the
 # logic in llvm_map_components_to_libnames is order dependent on the target
 # libraries being created.
 set_property(GLOBAL PROPERTY LLVM_TARGETS_CONFIGURED On)