rustc::driver: Capitalize structs and enums

driver::session::crate_metadata is unused; removed.
rustc::metadata: Remove trait FileSearch
syntax::ast: Remove/Recover tests

`xorpush_test` and `test_marksof` are at `syntax::ast_util`.

Fixes #7952
auto merge of #11553 : klutzy/rust/rustc-cleanups, r=alexcrichton