auto merge of #15584 : alexcrichton/rust/warn-annoyances, r=cmr

* Don't warn about `#[crate_name]` if `--crate-name` is specified
* Don't warn about non camel case identifiers on `#[repr(C)]` structs
* Switch `mode` to `mode_t` in libc.

diff --cc src/liblibc/lib.rs
Simple merge

diff --cc src/librustc/lint/builtin.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge