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.

SPIRV/GlslangToSpv.cpp

@@ -1720,6 +1720,12 @@ void TGlslangToSpvTraverser::visitSymbol(glslang::TIntermSymbol* symbol)
     if (symbol->getType().getQualifier().isSpecConstant())
         spec_constant_op_mode_setter.turnOnSpecConstantOpMode();
 
+#ifdef ENABLE_HLSL
+    // Skip symbol handling if it is string-typed
+    if (symbol->getBasicType() == glslang::EbtString)
+        return;
+#endif
+
     // getSymbolId() will set up all the IO decorations on the first call.
     // Formal function parameters were mapped during makeFunctions().
     spv::Id id = getSymbolId(symbol);