Make the host endianness check an integer constant expression.

I will remove the isBigEndianHost function once I update clang.

The ifdef logic is designed to
* not use configure/cmake to avoid breaking -arch i686 -arch ppc.
* default to little endian
* be as small as possible

It looks like sys/endian.h is the preferred header on most modern BSD systems,
but it is better to change this in a followup patch as machine/endian.h is
available on FreeBSD, OpenBSD, NetBSD and OS X.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179527 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/Hashing.h

@@ -151,7 +151,7 @@ namespace detail {
 inline uint64_t fetch64(const char *p) {
   uint64_t result;
   memcpy(&result, p, sizeof(result));
-  if (sys::isBigEndianHost())
+  if (sys::IsBigEndianHost)
     return sys::SwapByteOrder(result);
   return result;
 }
@@ -159,7 +159,7 @@ inline uint64_t fetch64(const char *p) {
 inline uint32_t fetch32(const char *p) {
   uint32_t result;
   memcpy(&result, p, sizeof(result));
-  if (sys::isBigEndianHost())
+  if (sys::IsBigEndianHost)
     return sys::SwapByteOrder(result);
   return result;
 }

include/llvm/Support/Endian.h

@@ -37,7 +37,7 @@ namespace detail {
 namespace endian {
 template<typename value_type, endianness endian>
 inline value_type byte_swap(value_type value) {
-  if (endian != native && sys::isBigEndianHost() != (endian == big))
+  if (endian != native && sys::IsBigEndianHost != (endian == big))
     return sys::SwapByteOrder(value);
   return value;
 }

include/llvm/Support/Host.h

@@ -15,22 +15,30 @@
 #define LLVM_SUPPORT_HOST_H
 
 #include "llvm/ADT/StringMap.h"
+
+#if defined(__linux__)
+#include <endian.h>
+#else
+#ifndef _MSC_VER
+#include <machine/endian.h>
+#endif
+#endif
+
 #include <string>
 
 namespace llvm {
 namespace sys {
 
-  inline bool isLittleEndianHost() {
-    union {
-      int i;
-      char c;
-    };
-    i = 1;
-    return c;
-  }
+#if BYTE_ORDER == BIG_ENDIAN
+  static const bool IsBigEndianHost = true;
+#else
+  static const bool IsBigEndianHost = false;
+#endif
+
+  static const bool IsLittleEndianHost = !IsBigEndianHost;
 
   inline bool isBigEndianHost() {
-    return !isLittleEndianHost();
+    return IsBigEndianHost;
   }
 
   /// getDefaultTargetTriple() - Return the default target triple the compiler