CMake: allow overriding CMAKE_CXX_VISIBILITY_PRESET

[sugak]

Summary

This allows overriding the CMAKE_CXX_VISIBILITY_PRESET on the command line. For example, setting the value to default lets PIC LLVM static libraries be converted to DSOs, without the need to rebuild LLVM with BUILD_SHARED_LIBS=ON.

[sugak]

friendly ping

[wenlei]

Makes sense to me to still allow overriding. Thanks for the change.

[wenlei]

This is blocking our internal adoption of llvm-12, going to commit on behalf of Igor now.

llvm/lib/Target/CMakeLists.txt

add_llvm_component_library(LLVMTarget
   Analysis
   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)
+if (NOT BUILD_SHARED_LIBS AND NOT APPLE 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)