We typically use macros like _LIBCPP_ABI_XYZ for ABI settings that can
be configured by the user. Since _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS is
not one of those, rename it to make it clearer it's an ABI tag
attribute.
As a drive-by remove the definition in the frozen C++03 config header
since it should never be needed (Ranges are not provided in C++03).