Improve the missing_abi lint.

compiler/rustc_ast/src/ast.rs

@@ -2810,6 +2810,8 @@ pub struct ModSpans {
 /// E.g., `extern { .. }` or `extern "C" { .. }`.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct ForeignMod {
+    /// Span of the `extern` keyword.
+    pub extern_span: Span,
     /// `unsafe` keyword accepted syntactically for macro DSLs, but not
     /// semantically by Rust.
     pub safety: Safety,