[Darwin][Driver][clang] Prioritise -isysroot over --sysroot consistently

On Darwin, `clang` currently prioritises `-isysroot` over `--sysroot`
when selecting headers, but does the reverse when setting `-syslibroot`
for the linker, which determines library selection.

This is wrong, because it leads to using headers that are of different
versions from the libraries being linked.

We can see this from:

    ❯ bin/clang -v -xc - -o /dev/null \
        -isysroot /Applications/Xcode-14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.sdk \
        --sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.sdk <<<'int main(){}'
    clang version 20.0.0git (https://github.com/llvm/llvm-project.git 3de5dbb)
    Target: arm64-apple-darwin24.1.0
    [snip]
    #include "..." search starts here:
    #include <...> search starts here:
     /Users/carlocab/github/llvm-project/build-clang-config/lib/clang/20/include
     /Applications/Xcode-14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.sdk/usr/include
     /Applications/Xcode-14.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.sdk/System/Library/Frameworks (framework directory)
    End of search list.
     "/usr/bin/ld" -demangle -lto_library /Users/carlocab/github/llvm-project/build-clang-config/lib/libLTO.dylib -no_deduplicate -dynamic -arch arm64 -platform_version macos 13.3.0 13.3 -syslibroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.sdk -mllvm -enable-linkonceodr-outlining -o /dev/null /var/folders/nj/9vfk4f0n377605jhxxmngd8h0000gn/T/--b914c6.o -lSystem

We can fix this by reversing the order in which the `-syslibroot` flag
is determined.

Downstream bug report: Homebrew/homebrew-core#197277

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

Author: carlocab

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -428,15 +428,14 @@ void darwin::Linker::AddLinkArgs(Compilation &C, const ArgList &Args,
   Args.AddAllArgs(CmdArgs, options::OPT_sub__library);
   Args.AddAllArgs(CmdArgs, options::OPT_sub__umbrella);
 
-  // Give --sysroot= preference, over the Apple specific behavior to also use
-  // --isysroot as the syslibroot.
-  StringRef sysroot = C.getSysRoot();
-  if (sysroot != "") {
-    CmdArgs.push_back("-syslibroot");
-    CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
-  } else if (const Arg *A = Args.getLastArg(options::OPT_isysroot)) {
+  // Prioritise -isysroot to ensure that the libraries we link to are taken
+  // from the same SDK that our headers come from.
+  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

@@ -11,9 +11,9 @@
 // RUN: FileCheck --check-prefix=CHECK-APPLE-ISYSROOT < %t2 %s
 // CHECK-APPLE-ISYSROOT: "-arch" "i386"{{.*}} "-syslibroot" "{{[^"]*}}/FOO"
 
-// Check that honor --sysroot= over -isysroot, for Apple Darwin.
+// Check that honor -isysroot over --sysroot=, for Apple Darwin.
 // RUN: touch %t3.o
 // RUN: %clang -target i386-apple-darwin10 \
 // RUN:   -isysroot /FOO --sysroot=/BAR -### %t3.o 2> %t3
 // RUN: FileCheck --check-prefix=CHECK-APPLE-SYSROOT < %t3 %s
-// CHECK-APPLE-SYSROOT: "-arch" "i386"{{.*}} "-syslibroot" "{{[^"]*}}/BAR"
+// CHECK-APPLE-SYSROOT: "-arch" "i386"{{.*}} "-syslibroot" "{{[^"]*}}/FOO"