Point References to Top-Level Continuations to the first BB-Like Continuation

[CodaFi]

The GIR Parser currently requires the entry block be named the same as the function, but the GIRWriter renumbers it as bb0 anyways. While parsing, references to the function name should just point at the top-level continuation.

@foo : A -> B -> C -> ... -> _ {
bb0(%0 : A ; %1 : B ; %2 : C ; ... ; %return : (Z -> _)):
 -- etc.

bbN:
  %k = function_ref @foo -- points to bb0
}