[Bug]: `isIntrinsicType` returns true for types that extend an intrinsic.

### Describe the bug

The function `isIntrinsicType` (undocumented) in the compiler returns `true` for any type that _is assignable to_ the STL scalar of the same name. This is confusing because the function name implies that it returns true if the input type _IS_ the intrinsic type, not an extension of or assignable to it.

This is not a compiler public API function.

### Reproduction

```ts
const { A } = await runner.compile(`
@test
scalar A extends string;
`);

isIntrinsicType(runner.program, A, "string"); // true!
```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.