Make the func name optional for compiler/linker crashes

[mvdan]

If we're after a compiler or linker crash, we don't care about the resulting binary or how to run it. Thus the func name or entry point is not at all useful.

We can't just remove it now because we depend on it to find what ast.File to walk. We should instead just walk the ast.Package.

Split off #15.

[mvdan]

Also, swapping the package name to main and generating func main might break the reduction of programs that reproduce compiler bugs - see #15.

We shouldn't do this either.