[libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR (#85756)

In other contexts, install directories such as
LIBCXX_INSTALL_LIBRARY_DIR and LIBCXX_INSTALL_MODULES_DIR can be
specified either as a relative path, relative to CMAKE_INSTALL_PREFIX,
or as an absolute path.

When calculating the relative path between the two, account for the fact
that LIBCXX_INSTALL_MODULES_DIR and LIBCXX_INSTALL_LIBRARY_DIR can be
absolute paths too.