[Support] Reduce Dependence on Host.h

The intention behind this commit is to reduce the use of Host.h/Host.cpp
in Support, to where it is only necessary.

In this case, the endian-detection and support functionality needed by
these implementations can be provided by `Support/SwapByteOrder.h` in a
cleaner manner.

This patch also changes the byte swap in SHA256.cpp to use the byte swap
function from that header, rather than an inlined implementation.

Differential Revision: https://reviews.llvm.org/D137834

llvm/include/llvm/Support/OnDiskHashTable.h

@@ -17,7 +17,6 @@
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/EndianStream.h"
-#include "llvm/Support/Host.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>

llvm/lib/Support/FoldingSet.cpp

@@ -15,8 +15,8 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/ErrorHandling.h"
-#include "llvm/Support/Host.h"
 #include "llvm/Support/MathExtras.h"
+#include "llvm/Support/SwapByteOrder.h"
 #include <cassert>
 #include <cstring>
 using namespace llvm;

llvm/lib/Support/SHA1.cpp

@@ -18,15 +18,11 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Endian.h"
-#include "llvm/Support/Host.h"
+#include "llvm/Support/SwapByteOrder.h"
 #include <string.h>
 
 using namespace llvm;
 
-#if defined(BYTE_ORDER) && defined(BIG_ENDIAN) && BYTE_ORDER == BIG_ENDIAN
-#define SHA_BIG_ENDIAN
-#endif
-
 static inline uint32_t rol(uint32_t Number, int Bits) {
   return (Number << Bits) | (Number >> (32 - Bits));
 }
@@ -192,11 +188,10 @@ void SHA1::hashBlock() {
 }
 
 void SHA1::addUncounted(uint8_t Data) {
-#ifdef SHA_BIG_ENDIAN
+  if constexpr (sys::IsBigEndianHost)
     InternalState.Buffer.C[InternalState.BufferOffset] = Data;
-#else
+  else
     InternalState.Buffer.C[InternalState.BufferOffset ^ 3] = Data;
-#endif
 
   InternalState.BufferOffset++;
   if (InternalState.BufferOffset == BLOCK_LENGTH) {
@@ -267,20 +262,17 @@ void SHA1::final(std::array<uint32_t, HASH_LENGTH / 4> &HashResult) {
   // Pad to complete the last block
   pad();
 
-#ifdef SHA_BIG_ENDIAN
+  if constexpr (sys::IsBigEndianHost) {
     // Just copy the current state
     for (int i = 0; i < 5; i++) {
       HashResult[i] = InternalState.State[i];
     }
-#else
+  } else {
     // Swap byte order back
     for (int i = 0; i < 5; i++) {
-    HashResult[i] = (((InternalState.State[i]) << 24) & 0xff000000) |
-                    (((InternalState.State[i]) << 8) & 0x00ff0000) |
-                    (((InternalState.State[i]) >> 8) & 0x0000ff00) |
-                    (((InternalState.State[i]) >> 24) & 0x000000ff);
+      HashResult[i] = sys::getSwappedBytes(InternalState.State[i]);
+    }
   }
-#endif
 }
 
 std::array<uint8_t, 20> SHA1::final() {

llvm/lib/Support/SHA256.cpp

@@ -23,15 +23,11 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Endian.h"
-#include "llvm/Support/Host.h"
+#include "llvm/Support/SwapByteOrder.h"
 #include <string.h>
 
 namespace llvm {
 
-#if defined(BYTE_ORDER) && defined(BIG_ENDIAN) && BYTE_ORDER == BIG_ENDIAN
-#define SHA_BIG_ENDIAN
-#endif
-
 #define SHR(x, c) ((x) >> (c))
 #define ROTR(x, n) (((x) >> n) | ((x) << (32 - (n))))
 
@@ -171,11 +167,10 @@ void SHA256::hashBlock() {
 }
 
 void SHA256::addUncounted(uint8_t Data) {
-#ifdef SHA_BIG_ENDIAN
+  if constexpr (sys::IsBigEndianHost)
     InternalState.Buffer.C[InternalState.BufferOffset] = Data;
-#else
+  else
     InternalState.Buffer.C[InternalState.BufferOffset ^ 3] = Data;
-#endif
 
   InternalState.BufferOffset++;
   if (InternalState.BufferOffset == BLOCK_LENGTH) {
@@ -247,20 +242,17 @@ void SHA256::final(std::array<uint32_t, HASH_LENGTH / 4> &HashResult) {
   // Pad to complete the last block
   pad();
 
-#ifdef SHA_BIG_ENDIAN
+  if constexpr (sys::IsBigEndianHost) {
     // Just copy the current state
     for (int i = 0; i < 8; i++) {
       HashResult[i] = InternalState.State[i];
     }
-#else
+  } else {
     // Swap byte order back
     for (int i = 0; i < 8; i++) {
-    HashResult[i] = (((InternalState.State[i]) << 24) & 0xff000000) |
-                    (((InternalState.State[i]) << 8) & 0x00ff0000) |
-                    (((InternalState.State[i]) >> 8) & 0x0000ff00) |
-                    (((InternalState.State[i]) >> 24) & 0x000000ff);
+      HashResult[i] = sys::getSwappedBytes(InternalState.State[i]);
+    }
   }
-#endif
 }
 
 std::array<uint8_t, 32> SHA256::final() {