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

[BertalanD]

Summary

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. 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)

[thakis]

Nice!

lld/MachO/Writer.cpp

for (OutputSection *osec : seg->getSections()) {
       if (isThreadLocalData(osec->flags)) {
         if (!firstTLVDataSection)
           firstTLVDataSection = osec;
         osec->align = tlvAlign;
       }
 
       if (!isecPriorities.empty()) {
         if (auto *merged = dyn_cast<ConcatOutputSection>(osec)) {
-          llvm::stable_sort(merged->inputs,
-                            [&](InputSection *a, InputSection *b) {
-                              return isecPriorities[a] > isecPriorities[b];
-                            });
+          llvm::stable_sort(
+              merged->inputs, [&](InputSection *a, InputSection *b) {
+                return isecPriorities.lookup(a) > isecPriorities.lookup(b);
         }
       }
     }
   }
 }
 
 template <class LP> void Writer::createOutputSections() {
   TimeTraceScope timeScope("Create output sections");