Allow attributes on local function parameters

From discussion in LDM today (4/29/2019), this would help with async-iterator local functions that want to use `[EnumeratorCancellation]`.

We should also test other attributes:
- `[DoesNotReturn]`
- `[DoesNotReturnIf(bool)]`
- `[Disallow/Allow/Maybe/NotNull]`
- `[Maybe/NotNullWhen(bool)]`
- `[Obsolete]`
- ...

FYI @agocke 