JSRT APIs JsConvertValueToString and JsGetProperty fail if the input value is taggedInt instead of Object

We are incorrectly treating taggedInt as JavascriptNumber.

Same as [OS 6980708](https://microsoft.visualstudio.com/web/wi.aspx?pcguid=cb55739e-4afe-46a3-970f-1b49d8ee7564&id=6980708)

**Details from the bug above:**

I was trying to convert something to a string in an external function. Instead of calling JsConvertValueToString, I tried to get the "toString" method and call it.

```
MyExternalFunction(..., JsValueRef *pArguments, ...)
{
...
JsGetProperty(pArguments[...], propIdToString, &jsToString);
...
}
```

fails with `JsErrorArgumentNotObject` when I call it on `TaggedInt`.

I can do this for string conversion:
`host.externalFunction()`

I cannot do this:
`host.externalFunction(42)`