[lldb][cmake] Also use local submodule visibility on Darwin

[teemperor]

Summary

Currently building LLVM on macOS and on other platforms with LLVM_ENABLE_MODULES is using different module flags,
which means that a passing modules build on macOS might fail on Linux and vice versa. -fmodules-local-submodule-visibility
is the mode that has clearer semantics and is closer to the actual C++ module standard, so let's make this the default everywhere.

We can still test building without local submodule visibility on an additional bot by just changing the respective CMake flag. However,
if building without local-submodule-visibility breaks we won't revert other commits and we won't loose LLDB's/Clang's test run
information.

[Bigcheese]

lgtm

[aprantl]

That's great progress! Is the idea to make local submodule visibility the default on Darwin at some point int the future?

llvm/CMakeLists.txt

set(LLVM_TARGETS_TO_BUILD
    ${LLVM_EXPERIMENTAL_TARGETS_TO_BUILD})
 list(REMOVE_DUPLICATES LLVM_TARGETS_TO_BUILD)
 
 option(LLVM_ENABLE_PIC "Build Position-Independent Code" ON)
 option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
 option(LLVM_ENABLE_MODULES "Compile with C++ modules enabled." OFF)
 if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
   option(LLVM_ENABLE_MODULE_DEBUGGING "Compile with -gmodules." ON)
-  option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." OFF)
 else()
   option(LLVM_ENABLE_MODULE_DEBUGGING "Compile with -gmodules." OFF)
-  option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." ON)
 endif()
+option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." ON)
 option(LLVM_ENABLE_LIBCXX "Use libc++ if available." OFF)
 option(LLVM_STATIC_LINK_CXX_STDLIB "Statically link the standard library." OFF)
 option(LLVM_ENABLE_LLD "Use lld as C and C++ linker." OFF)
 option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LLVM_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 
 option(LLVM_ENABLE_DUMP "Enable dump functions even when assertions are disabled" OFF)