Auto merge of #65134 - davidtwco:issue-19834-improper-ctypes-in-extern-C-fn, r=rkruppe

improper_ctypes: `extern "C"` fns

cc #19834. Fixes #65867.

This pull request implements the change [described in this comment](https://github.com/rust-lang/rust/issues/19834#issuecomment-466671572).

cc @rkruppe @varkor @shepmaster

diff --cc src/librustc_codegen_llvm/back/write.rs
Simple merge