Move enum_variant_size_lint out of trans_crate().

[michaelwoerister]

The enum_variant_size_lint is located in trans because we formerly relied on LLVM for providing type layouts. This is not true any more and after collector-driven trans lands, enum_variant_size_lint is the only reason we walk the HIR in trans_crate() at all. This should be cleaned up.

[jonas-schievink]

Collector-driven trans is now merged, so I'm rebasing my patch for this.