[lld-macho] Parallelize linker optimization hint processing

This commit moves the parsing of linker optimization hints into
`ARM64::applyOptimizationHints`. This lets us avoid allocating memory
for holding the parsed information, and moves work out of
`ObjFile::parse`, which is not parallelized at the moment.

This change reduces the overhead of processing LOHs to 25-30 ms when
linking Chromium Framework on my M1 machine; previously it took close to
100 ms.

There's no statistically significant change in runtime for a --threads=1
link.

Performance figures with all 8 cores utilized:

      N           Min           Max        Median           Avg        Stddev
  x  20     3.8027232     3.8760762     3.8505335     3.8454145   0.026352574
  +  20     3.7019017     3.8660538     3.7546209     3.7620371   0.032680043
  Difference at 95.0% confidence
  	-0.0833775 +/- 0.019
  	-2.16823% +/- 0.494094%
  	(Student's t, pooled s = 0.0296854)

Differential Revision: https://reviews.llvm.org/D133439