[clang] [HLSL] Split out the ROV attribute from the resource attribute, make it a new spellable attribute. (PR #102414)

[Justin Bogner via cfe-commits]

https://github.com/bogner commented:

It'd be better to make the attribute take no arguments, and we can tell if something is ROV by its presence alone, so:
```hlsl
[[hlsl::is_rov]] __hlsl_resource_t handle;
```
would be an ROV resource, but
```hlsl
__hlsl_resource_t handle;
```
would not.

I think this also applies to the AST attribute - it only needs to be there when we have `IsROV == true`

https://github.com/llvm/llvm-project/pull/102414