rustc_metadata: use 0 in index::Index to indicate missing entries.
rustc_metadata: use 0 in index::Index to indicate missing entries.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust