[lld-macho] Add LTO cache support

This adds support for the lld-only `--thinlto-cache-policy` option, as well as
implementations for ld64's `-cache_path_lto`, `-prune_interval_lto`,
`-prune_after_lto`, and `-max_relative_cache_size_lto`.

Test is adapted from lld/test/ELF/lto/cache.ll

Differential Revision: https://reviews.llvm.org/D105922