Add support for thiscall calling convention.

Patch by Charles Davis and Steven Watanabe!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103902 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/CBackend/CBackend.cpp

@@ -2165,6 +2165,9 @@ void CWriter::printFunctionSignature(const Function *F, bool Prototype) {
    case CallingConv::X86_FastCall:
     Out << "__attribute__((fastcall)) ";
     break;
+   case CallingConv::X86_ThisCall:
+    Out << "__attribute__((thiscall)) ";
+    break;
    default:
     break;
   }