HLSL: Add support for printf().

Translate printf() to what GL_EXT_debug_printf has done. HLSL could
define non-constant string variable and we don't have such features
in SPIR-V, so just support constant string variable.

glslang/MachineIndependent/intermOut.cpp

@@ -1321,6 +1321,9 @@ static void OutputConstantUnion(TInfoSink& out, const TIntermTyped* node, const
                 out.debug << buf << "\n";
             }
             break;
+        case EbtString:
+            out.debug << "\"" << constUnion[i].getSConst()->c_str() << "\"\n";
+            break;
         default:
             out.info.message(EPrefixInternalError, "Unknown constant", node->getLoc());
             break;