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