Convert two rustc_middle::lint functions to Span methods.

`rustc_middle` is a huge crate and it's always good to move stuff out of
it. There are lots of similar methods already on `Span`, so these two
functions, `in_external_macro` and `is_from_async_await`, fit right in.
The diff is big because `in_external_macro` is used a lot by clippy
lints.

compiler/rustc_span/src/lib.rs

@@ -600,11 +600,43 @@ impl Span {
         !self.is_dummy() && sm.is_span_accessible(self)
     }
 
+    /// Returns whether `span` originates in a foreign crate's external macro.
+    ///
+    /// This is used to test whether a lint should not even begin to figure out whether it should
+    /// be reported on the current node.
+    pub fn in_external_macro(self, sm: &SourceMap) -> bool {
+        let expn_data = self.ctxt().outer_expn_data();
+        match expn_data.kind {
+            ExpnKind::Root
+            | ExpnKind::Desugaring(
+                DesugaringKind::ForLoop
+                | DesugaringKind::WhileLoop
+                | DesugaringKind::OpaqueTy
+                | DesugaringKind::Async
+                | DesugaringKind::Await,
+            ) => false,
+            ExpnKind::AstPass(_) | ExpnKind::Desugaring(_) => true, // well, it's "external"
+            ExpnKind::Macro(MacroKind::Bang, _) => {
+                // Dummy span for the `def_site` means it's an external macro.
+                expn_data.def_site.is_dummy() || sm.is_imported(expn_data.def_site)
+            }
+            ExpnKind::Macro { .. } => true, // definitely a plugin
+        }
+    }
+
     /// Returns `true` if `span` originates in a derive-macro's expansion.
     pub fn in_derive_expansion(self) -> bool {
         matches!(self.ctxt().outer_expn_data().kind, ExpnKind::Macro(MacroKind::Derive, _))
     }
 
+    /// Return whether `span` is generated by `async` or `await`.
+    pub fn is_from_async_await(self) -> bool {
+        matches!(
+            self.ctxt().outer_expn_data().kind,
+            ExpnKind::Desugaring(DesugaringKind::Async | DesugaringKind::Await),
+        )
+    }
+
     /// Gate suggestions that would not be appropriate in a context the user didn't write.
     pub fn can_be_used_for_suggestions(self) -> bool {
         !self.from_expansion()