[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