Change the treatment of default implementations on interfaces in JVM
compatibility mode. Previously, the IR backend moved the actual
default implementation to the DefaultImpls class, and then bridged to
it from the interface default. The old backend did the reverse, at the
cost of an additional accessor, in order to gain better binary
compatibility properties. See #2612 for discussion.
The accessor needs to call a specific implementation, so must be
performed through an `invokespecial`. We trick the
SyntheticAccessorLowering into doing this for us, by marking the
bridging call as a super call. We do this in want of an explicit
`invokespecial` Ir Node.
InterfaceDefaultCallsPhase previously assumed the old behaviour of the
IR backend (that calls to default implementations, e.g. `foo$default`
should target `DefaultImpls.foo$default`). But now the bridge to
foo$default resides on `DefaultImpls` already, causing that pass to
create a recursive loop. We cut that loop with a simple check.
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