[lld-macho] Parallelize scanning the symbol tables in export/unexport-ing.

[oontvoo]

Summary

(Split from D113167)
Benchmarking on one of our large apps which exports a few thousands symbols,
this showed an improvement of ~17%.

x ./LLD_no_parallel.txt
+ ./LLD_with_parallel.txt

    N           Min           Max        Median           Avg        Stddev
x  10         84.01         89.41         88.64        87.693     1.7424061
+  10          71.9         74.29         72.63        72.753    0.77734663
Difference at 95.0% confidence
	-14.94 +/- 1.26763
	-17.0367% +/- 1.44553%
	(Student's t, pooled s = 1.34912)

(wallclock)

[int3]

Thanks!

lld/MachO/Driver.cpp

if (config->timeTraceEnabled)
 
     // FIXME: should terminate the link early based on errors encountered so
     // far?
 
     createSyntheticSections();
     createSyntheticSymbols();
 
     if (!config->exportedSymbols.empty()) {
-      for (Symbol *sym : symtab->getSymbols()) {
+      parallelForEach(symtab->getSymbols(), [](Symbol *sym) {
         if (auto *defined = dyn_cast<Defined>(sym)) {
           StringRef symbolName = defined->getName();
           if (config->exportedSymbols.match(symbolName)) {
             if (defined->privateExtern) {
               warn("cannot export hidden symbol " + symbolName +
                    "\n>>> defined in " + toString(defined->getFile()));
             }
           } else {
             defined->privateExtern = true;
           }
         }
-      }
+      });
     } else if (!config->unexportedSymbols.empty()) {
-      for (Symbol *sym : symtab->getSymbols())
+      parallelForEach(symtab->getSymbols(), [](Symbol *sym) {
         if (auto *defined = dyn_cast<Defined>(sym))
           if (config->unexportedSymbols.match(defined->getName()))
             defined->privateExtern = true;
+      });
     }
 
     for (const Arg *arg : args.filtered(OPT_sectcreate)) {
       StringRef segName = arg->getValue(0);
       StringRef sectName = arg->getValue(1);
       StringRef fileName = arg->getValue(2);
       Optional<MemoryBufferRef> buffer = readFile(fileName);
       if (buffer)