[lld/mac] Add support for distributed ThinLTO

Adds support for the following flags:

* --thinlto-index-only, --thinlto-index-only=
* --thinlto-emit-imports-files
* --thinlto-emit-index-files
* --thinlto-object-suffix-replace=
* --thinlto-prefix-replace=

See https://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
for some words on --thinlto-index-only.

I don't really need the other flags, but they were in the vicinity
and _someone_ might need them, so I figured I'd add them too.

`-object_path_lto` now sets `c.AlwaysEmitRegularLTOObj` as in the other ports,
which means it can now only point to a filename for non-thin LTO.
I think that was the intent of D129705 anyways, so update
test/MachO/lto-object-path.ll to use a non-thin bitcode file for that test.

Differential Revision: https://reviews.llvm.org/D138451