[clang] [Darwin] Add reverse mappings for aarch64/aarch64_32 to darwin arch names

These are mapped in MachO::getMachOArchName already, but were missing
in ToolChain::getDefaultUniversalArchName.

Having these reverse mapped here fixes weird inconsistencies like
-dumpmachine showing a target triple like "aarch64-apple-darwin",
while "clang -target aarch64-apple-darwin" didn't use to work (ended
up mapped as unknown-apple-ios).

Differential Revision: https://reviews.llvm.org/D79117

clang/lib/Driver/ToolChain.cpp

@@ -230,9 +230,12 @@ ToolChain::getTargetAndModeFromProgramName(StringRef PN) {
 StringRef ToolChain::getDefaultUniversalArchName() const {
   // In universal driver terms, the arch name accepted by -arch isn't exactly
   // the same as the ones that appear in the triple. Roughly speaking, this is
-  // an inverse of the darwin::getArchTypeForDarwinArchName() function, but the
-  // only interesting special case is powerpc.
+  // an inverse of the darwin::getArchTypeForDarwinArchName() function.
   switch (Triple.getArch()) {
+  case llvm::Triple::aarch64:
+    return "arm64";
+  case llvm::Triple::aarch64_32:
+    return "arm64_32";
   case llvm::Triple::ppc:
     return "ppc";
   case llvm::Triple::ppc64:

clang/test/Driver/darwin-arm64-target.c

@@ -0,0 +1,3 @@
+// RUN: %clang -target aarch64-apple-darwin %s -miphoneos-version-min=8.0 -### 2>&1 | FileCheck %s
+
+// CHECK: "-cc1"{{.*}} "-triple" "arm64-apple-ios8.0.0"