[lld/mac] Parallelize code signature computation

[thakis]

Summary

According to ministat, this is a small but measurable speedup
(using the repro in PR56121):

N           Min           Max        Median           Avg        Stddev

x 10 3.7439518 3.7783802 3.7730219 3.7655502 0.012375226
+ 10 3.6149218 3.692198 3.6519327 3.6502951 0.025905601
Difference at 95.0% confidence

-0.115255 +/- 0.0190746
-3.06078% +/- 0.506554%
(Student's t, pooled s = 0.0203008)

(Without 858e8b17f7365, this change here to use parallelFor is an 18% speedup,
and doing 858e8b17f7365 on top of this change is just a 2.55 +/- 0.58% win.
Doing both results in a total speedup of 20.85 +/- 0.44%.)

[thakis]

Anyone interested in a 3% speedup? :)

[oontvoo]

LG - i'm surprised that this is *only* ~3% (which is still good), because when I did a profile a few months ago, the hashing/coding signing function was hottest (~30-50% of the total time) when linking Youtube.

lld/MachO/SyntheticSections.cpp

 #include "Symbols.h"
 
 #include "lld/Common/CommonLinkerContext.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/EndianStream.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/LEB128.h"
+#include "llvm/Support/Parallel.h"
 #include "llvm/Support/Path.h"
 
 #if defined(__APPLE__)
 #include <sys/mman.h>
 
 #define COMMON_DIGEST_FOR_OPENSSL
 #include <CommonCrypto/CommonDigest.h>
 #else

 uint64_t CodeSignatureSection::getRawSize() const {
   return allHeadersSize + getBlockCount() * hashSize;
 }
 
 void CodeSignatureSection::writeHashes(uint8_t *buf) const {
   // NOTE: Changes to this functionality should be repeated in llvm-objcopy's
   // MachOWriter::writeSignatureData.
   uint8_t *hashes = buf + fileOff + allHeadersSize;
-  for (uint64_t i = 0; i < getBlockCount(); ++i) {
+  parallelFor(0, getBlockCount(), [&](size_t i) {
     sha256(buf + i * blockSize,
            std::min(static_cast<size_t>(fileOff - i * blockSize), blockSize),
            hashes + i * hashSize);
-  }
+  });
 #if defined(__APPLE__)
   // This is macOS-specific work-around and makes no sense for any
   // other host OS. See https://openradar.appspot.com/FB8914231
   //
   // The macOS kernel maintains a signature-verification cache to
   // quickly validate applications at time of execve(2).  The trouble
   // is that for the kernel creates the cache entry at the time of the
   // mmap(2) call, before we have a chance to write either the code to