[Sema]Set IsSending flag to subscript.getter if needed

[stzn]

Resolves #79041

When using the sending keyword in a subscript parameter through protocol conformance, an assertion failure occurs when emitting SIL.

For example, run the following code with swift -frontend -emit-sil -swift-version 6:

class NonSendable {}
protocol P {
    subscript(_: sending NonSendable) -> Bool { get }
}

struct S: P {
    subscript(_: sending NonSendable) -> Bool { true }
}

It triggers the following assertion failure:

sil_witness_table hidden S: P module main {
  method #P.subscript!getter: <Self where Self : P> (Self) -> (Assertion failed: (flags.isSending() && "Only valid when sending is enabled"), function printParameterFlags, file ASTPrinter.cpp, line 3759.

It appears we need to add a flag when the subscript parameter includes the sending keyword.

[omochi]

@swift-ci please smoke test