Fix TargetParserTests for big-endian hosts (#152407)

The new `sys::detail::getHostCPUNameForARM` for Windows (#151596) was
implemented using a C++ bit-field, which caused the associated unit
tests to fail on big-endian machines as it assumed a little-endian
layout.

This change switches from the C++ bit-field to LLVM's `BitField` type
instead.

Author: dpaoliello

llvm/lib/TargetParser/Host.cpp

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/TargetParser/Host.h"
+#include "llvm/ADT/Bitfields.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
@@ -434,22 +435,14 @@ StringRef sys::detail::getHostCPUNameForARM(StringRef ProcCpuinfoContent) {
 StringRef sys::detail::getHostCPUNameForARM(uint64_t PrimaryCpuInfo,
                                             ArrayRef<uint64_t> UniqueCpuInfos) {
   // On Windows, the registry provides cached copied of the MIDR_EL1 register.
-  union MIDR_EL1 {
-    uint64_t Raw;
-    struct _Components {
-      uint64_t Revision : 4;
-      uint64_t Partnum : 12;
-      uint64_t Architecture : 4;
-      uint64_t Variant : 4;
-      uint64_t Implementer : 8;
-      uint64_t Reserved : 32;
-    } Components;
-  };
+  using PartNum = Bitfield::Element<uint16_t, 4, 12>;
+  using Implementer = Bitfield::Element<uint16_t, 24, 8>;
+  using Variant = Bitfield::Element<uint16_t, 20, 4>;
 
   SmallVector<std::string> PartsHolder;
   PartsHolder.reserve(UniqueCpuInfos.size());
   for (auto Info : UniqueCpuInfos)
-    PartsHolder.push_back("0x" + utohexstr(MIDR_EL1{Info}.Components.Partnum,
+    PartsHolder.push_back("0x" + utohexstr(Bitfield::get<PartNum>(Info),
                                            /*LowerCase*/ true,
                                            /*Width*/ 3));
 
@@ -459,14 +452,14 @@ StringRef sys::detail::getHostCPUNameForARM(uint64_t PrimaryCpuInfo,
     Parts.push_back(Part);
 
   return getHostCPUNameForARMFromComponents(
-      "0x" + utohexstr(MIDR_EL1{PrimaryCpuInfo}.Components.Implementer,
+      "0x" + utohexstr(Bitfield::get<Implementer>(PrimaryCpuInfo),
                        /*LowerCase*/ true,
                        /*Width*/ 2),
       /*Hardware*/ "",
-      "0x" + utohexstr(MIDR_EL1{PrimaryCpuInfo}.Components.Partnum,
+      "0x" + utohexstr(Bitfield::get<PartNum>(PrimaryCpuInfo),
                        /*LowerCase*/ true,
                        /*Width*/ 3),
-      Parts, [=]() { return MIDR_EL1{PrimaryCpuInfo}.Components.Variant; });
+      Parts, [=]() { return Bitfield::get<Variant>(PrimaryCpuInfo); });
 }
 
 namespace {