[CMake][llvm] add_llvm_library's functions require default visibility

[radford]

Summary

Compiling LLVM, as a cmake sub-project that compiles with visibility
hidden, will generate a "cannot export hidden symbol" error because we
require default visibility. Even though we export using
-Wl,-exported_symbols_list,libllvm.exports, if each .cpp file is
compiled with -fvisibility=hidden, then you'll get this warning and
linking will fail. Essentially -exported_symbols_list is not strong
enough to reverse the -fvisibility=hidden.

llvm/cmake/modules/AddLLVM.cmake

else()
     endif()
     set_property(GLOBAL APPEND PROPERTY LLVM_EXPORTS ${name})
   endif()
   if (ARG_MODULE)
     set_target_properties(${name} PROPERTIES FOLDER "Loadable modules")
   else()
     set_target_properties(${name} PROPERTIES FOLDER "Libraries")
   endif()
+  set_target_properties(${name} PROPERTIES CXX_VISIBILITY_PRESET default)
 endmacro(add_llvm_library name)
 
 macro(add_llvm_executable name)
   cmake_parse_arguments(ARG
     "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS"
     "ENTITLEMENTS;BUNDLE_PATH"
     "DEPENDS"
     ${ARGN})