Implement basic support for opaque parameters.
Implement function parameters of the form `some P` be synthesizing an
implicit generic parameter whose requirements come from the opaque
type. We then map the opaque type back to the generic parameter, and
print as the opaque type. This allows us to write functions with
implicit generic parameters:
```swift
func f(_: some Collection) { }
```
which is equivalent to:
```swift
func f<T: Collection>(_: some T) { }
```
where `T` is an otherwise-unused generic parameter name.
All of this is behind the experimental frontend flag
`-enable-experimental-opaque-parameters`.
