[Darwin toolchain] Tune the logic for finding arclite.

[doug.gregor]

Summary

The heuristic used to determine where the arclite libraries are to be
found was based on the path of the clang executable. However, in some
scenarios the clang executable is within a toolchain that does not
have arclite. When this happens, derive the arclite paths from the
sysroot option.

This allows Clang to correctly derive the arclite directory in, e.g.,
Swift CI, using similar logic to what the Swift driver has been doing
for several years.

clang/lib/Driver/ToolChains/Darwin.cpp

void DarwinClang::AddLinkARCArgs(const ArgList &Args,
 
   if ((runtime.hasNativeARC() || !isObjCAutoRefCount(Args)) &&
       runtime.hasSubscripting())
     return;
 
   SmallString<128> P(getDriver().ClangExecutable);
   llvm::sys::path::remove_filename(P); // 'clang'
   llvm::sys::path::remove_filename(P); // 'bin'
+  llvm::sys::path::append(P, "lib", "arc");
 
   // 'libarclite' usually lives in the same toolchain as 'clang'. However, the
   // Swift open source toolchains for macOS distribute Clang without libarclite.
   // In that case, to allow the linker to find 'libarclite', we point to the
   // 'libarclite' in the XcodeDefault toolchain instead.
-  if (getXcodeDeveloperPath(P).empty()) {
-    if (const Arg *A = Args.getLastArg(options::OPT_isysroot)) {
+  if (!getVFS().exists(P)) {
+    auto updatePath = [&](const Arg *A) {
       // Try to infer the path to 'libarclite' in the toolchain from the
       // specified SDK path.
       StringRef XcodePathForSDK = getXcodeDeveloperPath(A->getValue());
-      if (!XcodePathForSDK.empty()) {
-        P = XcodePathForSDK;
-        llvm::sys::path::append(P, "Toolchains/XcodeDefault.xctoolchain/usr");
-      }
+      if (XcodePathForSDK.empty())
+        return false;
+
+      llvm::sys::path::append(P, "Toolchains/XcodeDefault.xctoolchain/usr",
+                              "lib", "arc");
+      return getVFS().exists(P);
+    };
+
+    bool updated = false;
+    if (const Arg *A = Args.getLastArg(options::OPT_isysroot))
+      updated = updatePath(A);
+
+    if (!updated) {
+      if (const Arg *A = Args.getLastArg(options::OPT__sysroot_EQ))
+        updatePath(A);
     }
   }
 
   CmdArgs.push_back("-force_load");
-  llvm::sys::path::append(P, "lib", "arc", "libarclite_");
+  llvm::sys::path::append(P, "libarclite_");
   // Mash in the platform.
   if (isTargetWatchOSSimulator())
     P += "watchsimulator";
   else if (isTargetWatchOS())
     P += "watchos";
   else if (isTargetTvOSSimulator())
     P += "appletvsimulator";
   else if (isTargetTvOS())