[lld/mac] Use fewer magic numbers in magic $ld$ handling code

[thakis]

Summary

Also simply a conditional and de-alias a variable.
Minor cleanups, no behavior change.

[thakis]

lld/MachO/InputFiles.cpp

978

(can't consume_front here since we want to pass on the full originalName below)

[thakis]

more

[alexander-shaposhnikov]

lld/MachO/InputFiles.cpp

1052–1053

https://llvm.org/docs/CodingStandards.html#use-early-exits-and-continue-to-simplify-code
e.g. the "preferred style" also uses extra lines for early returns with the benefit of keeping the code path where the real changes take effect at the same indentation level. Here the code is small enough, personally I don't have a very strong opinion regarding different versions.

lld/MachO/InputFiles.cpp

if (is_contained(skipPlatformChecks, intfRef.getInstallName()) ||
       loadReexport(intfRef.getInstallName(), exportingFile, topLevel);
   }
 }
 
 // $ld$ symbols modify the properties/behavior of the library (e.g. its install
 // name, compatibility version or hide/add symbols) for specific target
 // versions.
 bool DylibFile::handleLDSymbol(StringRef originalName) {
   if (!originalName.startswith("$ld$"))
     return false;
 
   StringRef action;
   StringRef name;
-  std::tie(action, name) = originalName.drop_front(4 /* $ld$ */).split('$');
+  std::tie(action, name) = originalName.drop_front(strlen("$ld$")).split('$');
   if (action == "previous")
     handleLDPreviousSymbol(name, originalName);
   else if (action == "install_name")
     handleLDInstallNameSymbol(name, originalName);
   return true;
 }
 
 void DylibFile::handleLDPreviousSymbol(StringRef name, StringRef originalName) {
   // originalName: $ld$ previous $ <installname> $ <compatversion> $
   // <platformstr> $ <startversion> $ <endversion> $ <symbol-name> $
   StringRef installName;
   StringRef compatVersion;
   StringRef platformStr;
   StringRef startVersion;
   StringRef endVersion;
   StringRef symbolName;
   StringRef rest;
 
   std::tie(installName, name) = name.split('$');
   std::tie(compatVersion, name) = name.split('$');
   std::tie(platformStr, name) = name.split('$');
   std::tie(startVersion, name) = name.split('$');
-  std::tie(endVersion, symbolName) = name.split('$');
-  std::tie(symbolName, rest) = symbolName.split('$');
+  std::tie(endVersion, name) = name.split('$');
+  std::tie(symbolName, rest) = name.split('$');
   // TODO: ld64 contains some logic for non-empty symbolName as well.
   if (!symbolName.empty())
     return;
   unsigned platform;
   if (platformStr.getAsInteger(10, platform) ||
       platform != static_cast<unsigned>(config->platform()))
     return;
 

 }
 
 void DylibFile::handleLDInstallNameSymbol(StringRef name,
                                           StringRef originalName) {
   // originalName: $ld$ install_name $ os<version> $ install_name
   StringRef condition, installName;
   std::tie(condition, installName) = name.split('$');
   VersionTuple version;
-  if (!condition.startswith("os") ||
-      version.tryParse(condition.drop_front(2 /* os */))) {
+  if (!condition.consume_front("os") || version.tryParse(condition))
     warn("failed to parse os version, symbol '" + originalName + "' ignored");
-    return;
-  }
-  if (version != config->platformInfo.minimum)
-    return;
+  else if (version == config->platformInfo.minimum)
     dylibName = saver.save(installName);
 }
 
 ArchiveFile::ArchiveFile(std::unique_ptr<object::Archive> &&f)
     : InputFile(ArchiveKind, f->getMemoryBufferRef()), file(std::move(f)) {
   for (const object::Archive::Symbol &sym : file->symbols())
     symtab->addLazy(sym.getName(), this, sym);
 }