compiler: generalize cleanup of unresolved placeholder pointer types

compiler: generalize cleanup of unresolved placeholder pointer types

    This change extends the work in https://golang.org/cl/51131 to include
    placeholder pointer types created for Go function types, which can
    also be left dangling/unresolved in some instances. This fixes an
    assert in Llvm_backend::materializeComposite.

    Test case can be found in https://golang.org/cl/191743.

    Updates golang/go#33020.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/191744

From-SVN: r274935