[lldb] Fix -Wreturn-type in RegisterInfos_x86_64_with_base_shared.cpp (NFC)

/data/llvm-project/lldb/source/Plugins/Process/Utility/RegisterInfos_x86_64_with_base_shared.cpp:319:1: error: non-void function does
not return a value in all control paths [-Werror,-Wreturn-type]
}
^
1 error generated.