C-Smith: use temp files instead of local files
C-Smith: use temp files instead of local files

This allows us to have multiple processes generating C files with C-Smith and
testing them on bindgen without stepping on each others' toes.

emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen