[lld-macho][nfc] Factor out callgraph parsing code

`parseSections()` is a getting a bit large unwieldy, let's factor out
logic where we can.

Other minor changes in this diff:
* `"__cg_profile"` is now a global constexpr
* We now use `checkError()` instead of `fatal()`-ing without handling
  the Error
* Check for `callGraphProfileSort` before checking the section name,
  since the boolean comparison is likely cheaper

Reviewed By: #lld-macho, lgrey, oontvoo

Differential Revision: https://reviews.llvm.org/D119892