box_collection lint shouldn't trigger when used in `extern "C"` functions

Summary

Clippy warns about Box<Vec<T>> as being a useless heap allocation, but when they are used in extern "C" functions the Box is necessary as Vec's layout isn't repr(C).

Lint Name

box_collection

Reproducer

I tried this code:

#[no_mangle]
pub extern "C" fn vec_tag_new() -> Box<Vec<u32>> {
    Box::new(Vec::new())
}

I saw this happen:

error: you seem to be trying to use `Box<Vec<..>>`. Consider using just `Vec<..>`
   --> src/lib.rs:283:36
    |
283 | pub extern "C" fn vec_tag_new() -> Box<Vec<u32>> {
    |                                    ^^^^^^^^^^^^^
    |
    = note: `-D clippy::box-collection` implied by `-D warnings`
    = help: `Vec<..>` is already on the heap, `Box<Vec<..>>` makes an extra allocation
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#box_collection

I expected to see this happen:

No lint.

Version

rustc 1.59.0 (9d1b2106e 2022-02-23)
binary: rustc
commit-hash: 9d1b2106e23b1abd32fce1f17267604a5102f57a
commit-date: 2022-02-23
host: x86_64-apple-darwin
release: 1.59.0
LLVM version: 13.0.0

Additional Labels

No response

Start with the box_collection lint implementation and its existing tests, then inspect how the lint handles function signatures and extern "C" items. Reproduce the shown Box<Vec> case and add coverage showing that it is not linted in an extern "C" function while the usual case still is.