[CMake][clang] add_clang_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,libclang.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.

clang/cmake/modules/AddClang.cmake

if(TARGET ${lib})
       endif()
       set_property(GLOBAL APPEND PROPERTY CLANG_EXPORTS ${lib})
     else()
       # Add empty "phony" target
       add_custom_target(${lib})
     endif()
   endforeach()
 
+  set_target_properties(${name} PROPERTIES CXX_VISIBILITY_PRESET default)
   set_target_properties(${name} PROPERTIES FOLDER "Clang libraries")
   set_clang_windows_version_resource_properties(${name})
 endmacro(add_clang_library)
 
 macro(add_clang_executable name)
   add_llvm_executable( ${name} ${ARGN} )
   set_target_properties(${name} PROPERTIES FOLDER "Clang executables")
   set_clang_windows_version_resource_properties(${name})