Calling a default method from an inlined function doesn't work

[msullivan]

Right now, if we try to inline a function that makes a call to a default method, the compiler ICEs trying to look up the default method.

This is an artifact of the completely bogus way that I implemented cross crate default methods, in which each crate generates fresh fake def_ids for the default methods in all of the impls that they see. The right thing to do is to actually export default method information in metadata.

Part of #2794.