[ORC] Fix fallout from switch to _objc_map/load_images-based registration.

In f448d446 we switched to calling _objc_map_images and _objc_load_images
for MachO language metadata registration. This patch fixes some bugs arising
from that change:

(1) __objc_imageinfo processing was moved to a post-allocation pass, but this
    prevents us from discarding the redundant copies. This commit moves
    processing back to a pre-prune pass and inserts a symbol for the uniqued
    __objc_image section. Runtime objects use an edge pointing to this symbol
    to access the address.

(2) We were assuming that _objc_map_images & _objc_load_images were available
    in the Objective-C runtime on 10.15, but these functions didn't become
    available until later. This commit bumps the macOS version requirement to
    13.1 where the functions should be available.

(3) The ORC-RT trivial-swift-types-section.S test was missing an
    __objc_unwindinfo section, which triggered an assert that should have
    been an error. The assert has been turned into an error, and the testcase
    has been updated to include an __objc_imageinfo.

rdar://107846455