[Darwin][Driver][clang] Prioritise command line flags over DEFAULT_SYSROOT

If a toolchain is configured with `DEFAULT_SYSROOT`, then this could
result in an unintended value for `-syslibroot` being passed to the
linker if the user manually sets `-isysroot` or `SDKROOT`.

Let's fix this by prioritising command line flags when determining
`-syslibroot` before checking `getSysRoot`.

Downstream bug report: Homebrew/homebrew-core#197277

Co-authored-by: Bo Anderson <mail@boanderson.me>

Author: carlocab

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -430,13 +430,17 @@ void darwin::Linker::AddLinkArgs(Compilation &C, const ArgList &Args,
 
   // Give --sysroot= preference, over the Apple specific behavior to also use
   // --isysroot as the syslibroot.
-  StringRef sysroot = C.getSysRoot();
-  if (sysroot != "") {
+  // We check `OPT__sysroot_EQ` directly instead of `getSysRoot` to make sure we
+  // prioritise command line arguments over configuration of `DEFAULT_SYSROOT`.
+  if (const Arg *A = Args.getLastArg(options::OPT__sysroot_EQ)) {
     CmdArgs.push_back("-syslibroot");
-    CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
+    CmdArgs.push_back(A->getValue());
   } else if (const Arg *A = Args.getLastArg(options::OPT_isysroot)) {
     CmdArgs.push_back("-syslibroot");
     CmdArgs.push_back(A->getValue());
+  } else if (StringRef sysroot = C.getSysRoot(); sysroot != "") {
+    CmdArgs.push_back("-syslibroot");
+    CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
   }
 
   Args.AddLastArg(CmdArgs, options::OPT_twolevel__namespace);