Add Apple M4 host detection

Also add support for older ARM families (this is likely never going to
get used, since only macOS is officially supported as host OS, but nice
to have for completeness sake). Error handling (checking
CPUFAMILY_UNKNOWN) is also included here.

Finally, add links to extra documentation to make it easier for others
to update this in the future.

llvm/lib/TargetParser/Host.cpp

@@ -1460,6 +1460,18 @@ StringRef sys::getHostCPUName() {
   return getCPUNameFromS390Model(Id, HaveVectorSupport);
 }
 #elif defined(__APPLE__) && (defined(__arm__) || defined(__aarch64__))
+// Copied from <mach/machine.h> in the macOS SDK.
+//
+// Also available here, though usually not as up-to-date:
+// https://github.com/apple-oss-distributions/xnu/blob/xnu-11215.41.3/osfmk/mach/machine.h#L403-L452.
+#define CPUFAMILY_UNKNOWN 0
+#define CPUFAMILY_ARM_9 0xe73283ae
+#define CPUFAMILY_ARM_11 0x8ff620d8
+#define CPUFAMILY_ARM_XSCALE 0x53b005f5
+#define CPUFAMILY_ARM_12 0xbd1b0ae9
+#define CPUFAMILY_ARM_13 0x0cc90e64
+#define CPUFAMILY_ARM_14 0x96077ef1
+#define CPUFAMILY_ARM_15 0xa8511bca
 #define CPUFAMILY_ARM_SWIFT 0x1e2d6381
 #define CPUFAMILY_ARM_CYCLONE 0x37a09642
 #define CPUFAMILY_ARM_TYPHOON 0x2c91a47e
@@ -1471,13 +1483,45 @@ StringRef sys::getHostCPUName() {
 #define CPUFAMILY_ARM_FIRESTORM_ICESTORM 0x1b588bb3
 #define CPUFAMILY_ARM_BLIZZARD_AVALANCHE 0xda33d83d
 #define CPUFAMILY_ARM_EVEREST_SAWTOOTH 0x8765edea
+#define CPUFAMILY_ARM_IBIZA 0xfa33415e
+#define CPUFAMILY_ARM_PALMA 0x72015832
+#define CPUFAMILY_ARM_COLL 0x2876f5b5
+#define CPUFAMILY_ARM_LOBOS 0x5f4dea93
+#define CPUFAMILY_ARM_DONAN 0x6f5129ac
+#define CPUFAMILY_ARM_TAHITI 0x75d4acb9
+#define CPUFAMILY_ARM_TUPAI 0x204526d0
 
 StringRef sys::getHostCPUName() {
   uint32_t Family;
   size_t Length = sizeof(Family);
   sysctlbyname("hw.cpufamily", &Family, &Length, NULL, 0);
 
+  // This is found by testing on actual hardware, and by looking at:
+  // https://github.com/apple-oss-distributions/xnu/blob/xnu-11215.41.3/osfmk/arm/cpuid.c#L109-L231.
+  //
+  // Another great resource is
+  // https://github.com/AsahiLinux/docs/wiki/Codenames.
+  //
+  // NOTE: We choose to return `apple-mX` instead of `apple-aX`, since the M1,
+  // M2, M3 etc. aliases are more widely known to users than A14, A15, A16 etc.
+  // (and this code is basically only used on host macOS anyways).
   switch (Family) {
+  case CPUFAMILY_UNKNOWN:
+    return "invalid";
+  case CPUFAMILY_ARM_9:
+    return "arm920t"; // or arm926ej-s
+  case CPUFAMILY_ARM_11:
+    return "arm1136jf-s";
+  case CPUFAMILY_ARM_XSCALE:
+    return "xscale";
+  case CPUFAMILY_ARM_12:
+    return "invalid"; // Seems unsued by the kernel
+  case CPUFAMILY_ARM_13:
+    return "cortex-a8";
+  case CPUFAMILY_ARM_14:
+    return "cortex-a9";
+  case CPUFAMILY_ARM_15:
+    return "cortex-a7";
   case CPUFAMILY_ARM_SWIFT:
     return "swift";
   case CPUFAMILY_ARM_CYCLONE:
@@ -1495,14 +1539,28 @@ StringRef sys::getHostCPUName() {
   case CPUFAMILY_ARM_LIGHTNING_THUNDER:
     return "apple-a13";
   case CPUFAMILY_ARM_FIRESTORM_ICESTORM:
-    return "apple-m1";
+    return "apple-m1"; // A14 / M1
   case CPUFAMILY_ARM_BLIZZARD_AVALANCHE:
-    return "apple-m2";
+    return "apple-m2"; // A15 / M2
   case CPUFAMILY_ARM_EVEREST_SAWTOOTH:
-    return "apple-m3";
+    return "apple-m3"; // A16
+  case CPUFAMILY_ARM_IBIZA:
+    return "apple-m3"; // M3
+  case CPUFAMILY_ARM_PALMA:
+    return "apple-m3"; // M3 Max
+  case CPUFAMILY_ARM_COLL:
+    return "apple-m3"; // A17
+  case CPUFAMILY_ARM_LOBOS:
+    return "apple-m3"; // M3 Pro
+  case CPUFAMILY_ARM_DONAN:
+    return "apple-m4"; // M4
+  case CPUFAMILY_ARM_TAHITI:
+    return "apple-m4"; // A18 Pro
+  case CPUFAMILY_ARM_TUPAI:
+    return "apple-m4"; // A18
   default:
     // Default to the newest CPU we know about.
-    return "apple-m3";
+    return "apple-m4";
   }
 }
 #elif defined(_AIX)