[lld-macho] Parallel input section priorities for -order_file (#241470)

buildInputSectionPriorities() used to look up every symbol in
`priorities`, which does a bunch of work (hash a symbol, do some string
manipulation).

Instead, call getSymbolPriority in parallel over object files, and then
serially merge the results.

(This works because std::min() is associative.)

Takes a Chromium Framework link on my machine from 4.191 s ± 0.020 s to
3.970 s ± 0.045 s.

Chromium uses an order file with just a few entries. I also checked with
a synthetic order file that lists all symbols. There, this phase goes
from 781 ms to 357 ms, so it's still faster (the relative speedup is
just smaller).

Contributes to #222068, depends on #241367.

No behavior change.

GitOrigin-RevId: 82c882f2469f5ca80fa7bd75a42ec9a45d7774f4