Lint Abi in ast validation.

compiler/rustc_target/src/spec/abi.rs

@@ -87,6 +87,9 @@ pub fn all_names() -> Vec<&'static str> {
 }
 
 impl Abi {
+    /// Default ABI chosen for `extern fn` declarations without an explicit ABI.
+    pub const FALLBACK: Abi = Abi::C { unwind: false };
+
     #[inline]
     pub fn index(self) -> usize {
         // N.B., this ordering MUST match the AbiDatas array above.