[Driver] Properly report error for unsupported powerpc darwin/macos triples

The removal started at https://reviews.llvm.org/D50989 and
https://reviews.llvm.org/D75494 removed the Triple support. Without recognizing
Darwin triples as Mach-O, we will get assertion error in ToolChains/Darwin.h due
to the universal binary mechanism.

Fix llvm#47698

Author: MaskRay

clang/test/Driver/unsupported-target-arch.c

@@ -59,3 +59,7 @@
 // RUN: not %clang --target=thumbeb-none-elf -o %t.o %s 2> %t.err
 // RUN: FileCheck --input-file=%t.err --check-prefix=CHECK-THUMBEB-INVALID-ENV %s
 // CHECK-THUMBEB-INVALID-ENV: warning: mismatch between architecture and environment in target triple 'thumbeb-none-elf'; did you mean 'thumbeb-none-eabi'? [-Winvalid-command-line-argument]{{$}}
+
+// RUN: not %clang --target=powerpc-apple-darwin -o /dev/null %s 2> %t.err
+// RUN: FileCheck --input-file=%t.err --check-prefix=CHECK-PPCMAC %s
+// CHECK-PPCMAC: error: unknown target triple 'unknown-apple-macosx10.4.0'{{$}}

llvm/lib/TargetParser/Triple.cpp

@@ -782,6 +782,8 @@ static Triple::SubArchType parseSubArch(StringRef SubArchName) {
 }
 
 static Triple::ObjectFormatType getDefaultFormat(const Triple &T) {
+  if (T.isOSDarwin())
+    return Triple::MachO;
   switch (T.getArch()) {
   case Triple::UnknownArch:
   case Triple::aarch64:
@@ -790,9 +792,7 @@ static Triple::ObjectFormatType getDefaultFormat(const Triple &T) {
   case Triple::thumb:
   case Triple::x86:
   case Triple::x86_64:
-    if (T.isOSDarwin())
-      return Triple::MachO;
-    else if (T.isOSWindows())
+    if (T.isOSWindows())
       return Triple::COFF;
     return Triple::ELF;
 

llvm/unittests/TargetParser/TripleTest.cpp

@@ -1801,6 +1801,7 @@ TEST(TripleTest, FileFormat) {
   EXPECT_EQ(Triple::MachO, Triple("i686-apple-macosx").getObjectFormat());
   EXPECT_EQ(Triple::MachO, Triple("i686-apple-ios").getObjectFormat());
   EXPECT_EQ(Triple::MachO, Triple("i686---macho").getObjectFormat());
+  EXPECT_EQ(Triple::MachO, Triple("powerpc-apple-macosx").getObjectFormat());
 
   EXPECT_EQ(Triple::COFF, Triple("i686--win32").getObjectFormat());