Reapply [libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR (#86020)

This reapplies 272d1b44 (from #85756),
which was reverted in
40793703.

In the previous attempt, empty CMAKE_INSTALL_PREFIX was handled by
quoting them, in d209d134. That made the
calls to cmake_path(ABSOLUTE_PATH) succeed, but the output paths of that
weren't actually absolute, which was required by file(RELATIVE_PATH).

Avoid this issue by constructing a non-empty base directory variable
to use for calculating the relative path.