[lld-macho] Factor out common InputSection members

We have been creating many ConcatInputSections with identical values due
to .subsections_via_symbols. This diff factors out the identical values
into a Shared struct, to reduce memory consumption and make copying
cheaper.

I also changed `callSiteCount` from a uint32_t to a 31-bit field to save an
extra word.

All in all, this takes InputSection from 120 to 72 bytes (and
ConcatInputSection from 160 to 112 bytes), i.e. 30% size reduction in
ConcatInputSection.

Numbers for linking chromium_framework on my 3.2 GHz 16-Core Intel Xeon W:

      N           Min           Max        Median           Avg        Stddev
  x  20          4.14          4.24          4.18         4.183   0.027548999
  +  20          4.04          4.11         4.075        4.0775   0.018027756
  Difference at 95.0% confidence
          -0.1055 +/- 0.0149005
          -2.52211% +/- 0.356215%
          (Student's t, pooled s = 0.0232803)

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D105305