implement -msave-args in clang/llvm, like the sun did for gcc 

Index: lib/Target/X86/X86MachineFunctionInfo.h
--- lib/Target/X86/X86MachineFunctionInfo.h.orig
+++ lib/Target/X86/X86MachineFunctionInfo.h
@@ -45,6 +45,9 @@ class X86MachineFunctionInfo : public MachineFunctionI
   /// stack frame in bytes.
   unsigned CalleeSavedFrameSize = 0;
 
+  // SaveArgSize - Number of register arguments saved on the stack
+  unsigned SaveArgSize = 0;
+
   /// BytesToPopOnReturn - Number of bytes function pops on return (in addition
   /// to the space used by the return address).
   /// Used on windows platform for stdcall & fastcall name decoration
@@ -145,6 +148,9 @@ class X86MachineFunctionInfo : public MachineFunctionI
 
   unsigned getCalleeSavedFrameSize() const { return CalleeSavedFrameSize; }
   void setCalleeSavedFrameSize(unsigned bytes) { CalleeSavedFrameSize = bytes; }
+
+  unsigned getSaveArgSize() const { return SaveArgSize; }
+  void setSaveArgSize(unsigned bytes) { SaveArgSize = bytes; }
 
   unsigned getBytesToPopOnReturn() const { return BytesToPopOnReturn; }
   void setBytesToPopOnReturn (unsigned bytes) { BytesToPopOnReturn = bytes;}