[clang] Enhance handling of Apple-specific -arch/-target option values

The -arch option itself is Apple-specific, so, if -target is not passed
explicitely, we try to construct an Apple triple. If the default triple
is Apple-specific, we just update it's arch so it matches the one passed
via -arch option, otherwise, we construct a triple "from scratch".

The arm64e arch value is also Apple-specific, so, if a computed triple
has empty vendor name and/or OS and environment name, fill them
correspondingly.

Author: kovdan01

clang/lib/Driver/Driver.cpp

@@ -521,6 +521,41 @@ static llvm::Triple computeTargetTriple(const Driver &D,
   if (TargetTriple.contains("-unknown-gnu") || TargetTriple.contains("-pc-gnu"))
     Target.setOSName("hurd");
 
+  // Since -arch is an Apple-specific option, construct an Apple triple when
+  // -target is not explicitely passed.
+  if (Args.hasArg(options::OPT_arch) && !Args.hasArg(options::OPT_target)) {
+    if (Target.isOSBinFormatMachO()) {
+      // The default triple is already Apple-specific - just update the arch.
+      Target.setArchName(Args.getLastArg(options::OPT_arch)->getValue());
+    } else {
+      // The default triple is not Apple-specific - construct a new one to avoid
+      // handling unrelated info from the default one (e.g. environment).
+      Target = llvm::Triple(llvm::Triple::normalize(
+          Args.getLastArg(options::OPT_arch)->getValue()));
+      Target.setVendor(llvm::Triple::Apple);
+      // -fobjc-arc is not supported on versions of OS X prior to 10.6
+      if (Args.hasArg(options::OPT_fobjc_arc))
+        Target.setOSName("macosx10.6.0");
+      else
+        Target.setOS(llvm::Triple::Darwin);
+    }
+  }
+
+  // Since arm64e arch is Apple-specific, set VendorName and OS correspondingly
+  // if not set already.
+  if (!Args.hasArg(options::OPT_arch) && Target.getArchName() == "arm64e" &&
+      !Target.isOSBinFormatMachO()) {
+    if (Target.getVendorName().empty())
+      Target.setVendor(llvm::Triple::Apple);
+    if (Target.getOSAndEnvironmentName().empty()) {
+      // -fobjc-arc is not supported on versions of OS X prior to 10.6
+      if (Args.hasArg(options::OPT_fobjc_arc))
+        Target.setOSName("macosx10.6.0");
+      else
+        Target.setOS(llvm::Triple::Darwin);
+    }
+  }
+
   // Handle Apple-specific options available here.
   if (Target.isOSBinFormatMachO()) {
     // If an explicit Darwin arch name is given, that trumps all.