rustc: Don't panic on corrupt metadata

Fix a panic where the compiler was looking at stale or old metadata.

See #19798, #19772, #19757, #19744, #19718, #19691.
auto merge of #19799 : alexcrichton/rust/stop-panicking, r=huonw

Fix a panic where the compiler was looking at stale or old metadata.

See #19798, #19772, #19757, #19744, #19718, #19691.
Merge remote-tracking branch 'upstream/master'