[ConstraintSystem] NFC: Clarify why upward propagation of literal conformances is ncessary at the moment
This is NFC because only literal protocols are tracked at the moment.
Forward propagate (subtype -> supertype) only literal conformance
requirements since that helps solver to infer more types at
parameter positions.
```swift
func foo<T: ExpressibleByStringLiteral>(_: String, _: T) -> T {
fatalError()
}
func bar(_: Any?) {}
func test() {
bar(foo("", ""))
}
```
If one of the literal arguments doesn't propagate its
`ExpressibleByStringLiteral` conformance, we'd end up picking
`T` with only one type `Any?` which is incorrect.
This is not going to be necessary once bindings are filtered based
of requirements placed on a type variable.
swift
c03d7629 - [ConstraintSystem] NFC: Clarify why upward propagation of literal conformances is ncessary at the moment
