improve performance of getSDKName()

[aprantl]

Summary

The ".sdk" component is usually the last one in the -isysroot, so it makes more sense to scan from the back. Also, technically, someone could install Xcode into a directory ending with .sdk, which would break this heuristic.

[dblaikie]

@JDevlieghere it's an annoying bug in Phab, but if you approve a patch without providing any textual message, Phab doesn't send email to the mailing list - so it ends up looking like something's committed without approval (if you're just looking at the mailing list). So if possible, please include a message ("Thanks", "Looks good!" etc) when approving patches with Phab.

clang/lib/Driver/ToolChains/Darwin.cpp

     case DarwinPlatformKind::WatchOS:
       return "Watch";
   }
   llvm_unreachable("Unsupported platform");
 }
 
 StringRef Darwin::getSDKName(StringRef isysroot) {
   // Assume SDK has path: SOME_PATH/SDKs/PlatformXX.YY.sdk
-  auto BeginSDK = llvm::sys::path::begin(isysroot);
-  auto EndSDK = llvm::sys::path::end(isysroot);
+  auto BeginSDK = llvm::sys::path::rbegin(isysroot);
+  auto EndSDK = llvm::sys::path::rend(isysroot);
   for (auto IT = BeginSDK; IT != EndSDK; ++IT) {
     StringRef SDK = *IT;
     if (SDK.endswith(".sdk"))
       return SDK.slice(0, SDK.size() - 4);
   }
   return "";
 }