swift234

Sema: Fix crash calling protocol extension methods returning Self on existential

In r26737, Sema was changed to not wrap Self occurring in a protocol
extension in a DynamicSelf. The commit message was rather terse but
I believe this is because the metadata for Self is bound to the static
base type, not the runtime base type.

However, we still need to substitute Self in the return type for the
static base type in the case where the base is an existential,
otherwise we get an open existential type leaking out.

Also remove the default argument for replaceCovariantResultType(),
every call site passed in a value and it seems bad to omit it on
accident.

Fixes <rdar://problem/21433694>.

Swift SVN r29802