MachO: Add linker-optimisation hint framework to MC.

Another part of the ARM64 backend (so tests will be following soon).
This is currently used by the linker to relax adrp/ldr pairs into nops
where possible, though could well be more broadly applicable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205084 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCStreamer.h

@@ -19,6 +19,7 @@
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCDirectives.h"
 #include "llvm/MC/MCDwarf.h"
+#include "llvm/MC/MCLinkerOptimizationHint.h"
 #include "llvm/MC/MCWin64EH.h"
 #include "llvm/Support/DataTypes.h"
 #include <string>
@@ -466,6 +467,10 @@ public:
   ///
   virtual void EmitELFSize(MCSymbol *Symbol, const MCExpr *Value) = 0;
 
+  /// \brief Emit a Linker Optimization Hint (LOH) directive.
+  /// \param Args - Arguments of the LOH.
+  virtual void EmitLOHDirective(MCLOHType Kind, const MCLOHArgs &Args) {}
+
   /// EmitCommonSymbol - Emit a common symbol.
   ///
   /// @param Symbol - The common symbol to emit.