Explicitly store the set of input headers in the builder
Explicitly store the set of input headers in the builder

Rather than eagerly expanding multiple input headers into `-include <header>`
clang arguments, store them explicitly until we actually go to generate
bindings. Same deal for unsaved file contents.

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