[lld-macho] Fix & refactor symbol size calculations

I noticed two problems with the previous implementation:

* N_ALT_ENTRY symbols weren't being handled correctly -- they should
  determine the size of the previous symbol, even though they don't
  cause a new section to be created
* The last symbol in a section had its size calculated wrongly;
  the first subsection's size was used instead of the last one

I decided to take the opportunity to refactor things as well, mainly to
realize my observation
[here](https://reviews.llvm.org/D98837#inline-931511) that we could
avoid doing a binary search to match symbols with subsections. I think
the resulting code is a bit simpler too.

      N           Min           Max        Median           Avg        Stddev
  x  20          4.31          4.43          4.37        4.3775   0.034162922
  +  20          4.32          4.43          4.38        4.3755    0.02799906
  No difference proven at 95.0% confidence

Reviewed By: #lld-macho, alexshap

Differential Revision: https://reviews.llvm.org/D99972