Move endian constant from Host.h to SwapByteOrder.h, prune include

Works on this dependency chain:
  ArrayRef.h ->
  Hashing.h -> --CUT--
  Host.h ->
  StringMap.h / StringRef.h

ArrayRef is very popular, but Host.h is rarely needed. Move the
IsBigEndianHost constant to SwapByteOrder.h. Clients of that header are
more likely to need it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375316 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Driver/ToolChains/Arch/AArch64.cpp

@@ -12,6 +12,7 @@
 #include "clang/Driver/Options.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/TargetParser.h"
+#include "llvm/Support/Host.h"
 
 using namespace clang::driver;
 using namespace clang::driver::tools;

lib/Driver/ToolChains/Arch/ARM.cpp

@@ -13,6 +13,7 @@
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/TargetParser.h"
+#include "llvm/Support/Host.h"
 
 using namespace clang::driver;
 using namespace clang::driver::tools;

lib/Driver/ToolChains/Arch/PPC.cpp

@@ -13,6 +13,7 @@
 #include "clang/Driver/Options.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Option/ArgList.h"
+#include "llvm/Support/Host.h"
 
 using namespace clang::driver;
 using namespace clang::driver::tools;

lib/Driver/ToolChains/Arch/X86.cpp

@@ -13,6 +13,7 @@
 #include "clang/Driver/Options.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Option/ArgList.h"
+#include "llvm/Support/Host.h"
 
 using namespace clang::driver;
 using namespace clang::driver::tools;