[lld-macho] Don't create entries in isecPriorities during sorting (NFC)

If a value for a given key is not present, `DenseMap::operator[]`
default-constructs one, which is wasteful when we don't do anything with
it afterwards. Fix it by calling `lookup()` instead which only returns
the default value, but does not modify the map.

This speeds up linking a fair bit when only a small portion of all
sections are specified in the order file, like in the case of Chromium
Framework:

      N           Min           Max        Median           Avg        Stddev
  x  25      3.727684     3.8808699      3.753552     3.7702461     0.0397282
  +  25     3.6469049     3.7523289     3.6764321     3.6841622   0.025525047
  Difference at 95.0% confidence
  	-0.0860839 +/- 0.0189924
  	-2.28324% +/- 0.503745%
  	(Student's t, pooled s = 0.0333906)

Differential Revision: https://reviews.llvm.org/D134811