Add sending to subscript sending result

[stzn]

Resolves #79559

When the sending is applied to the return type in a subscript and SIL is emitted, the sending disappears from the output.

Here's an example to demonstrate this behavior:

class NonSendableKlass {}

struct S {
    subscript(_: sending NonSendableKlass) -> sending NonSendableKlass { NonSendableKlass() }
}

The SIL output(the result of emit-silgen) shows :

sil_stage raw
...
struct S {
  subscript(_: sending NonSendableKlass) -> NonSendableKlass { get } // no sending for the return value
  init()
}

...

sil hidden [ossa] @$s4main1SVyAA16NonSendableKlassCAEncig : $@convention(method) (@sil_sending @owned NonSendableKlass, S) -> @owned NonSendableKlass { // no @sil_sending for the return value

Note that while the parameter retains its sending in the SIL output, the return type's sending is missing.