[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 <[email protected]>

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);

clang/test/Driver/sysroot.c

@@ -4,10 +4,9 @@
 // CHECK-SYSROOTEQ: "-cc1"{{.*}} "-isysroot" "{{[^"]*}}/FOO"
 
 // Apple Darwin uses -isysroot as the syslib root, too.
-// We pass --sysroot="" to defeat any -DDEFAULT_SYSROOT parameter.
 // RUN: touch %t2.o
 // RUN: %clang -target i386-apple-darwin10 \
-// RUN:   -isysroot /FOO --sysroot="" -### %t2.o 2> %t2
+// RUN:   -isysroot /FOO -### %t2.o 2> %t2
 // RUN: FileCheck --check-prefix=CHECK-APPLE-ISYSROOT < %t2 %s
 // CHECK-APPLE-ISYSROOT: "-arch" "i386"{{.*}} "-syslibroot" "{{[^"]*}}/FOO"