librustc: Stop looking in metadata in type contents.

4x improvement in pre-trans compile time for rustc.
auto merge of #17647 : pcwalton/rust/stop-looking-in-metadata-in-tc, r=cmr

I don't know exactly what made this regress so badly…maybe it was all the lifetimes that @eddyb landed. According to `git blame` this is all somewhat old code. Regardless this is an enormous improvement in compile times.

r? @brson