Adjusted diagnostic output so that if there is no use in a item sequence,

then we just suggest the first legal position where you could inject a use.

To do this, I added `inject_use_span` field to `ModSpans`, and populate it in
parser (it is the span of the first token found after inner attributes, if any).
Then I rewrote the use-suggestion code to utilize it, and threw out some stuff
that is now unnecessary with this in place. (I think the result is easier to
understand.)

Then I added a test of issue 87613.

compiler/rustc_ast/src/ast.rs

@@ -2322,16 +2322,17 @@ pub enum ModKind {
     Unloaded,
 }
 
-#[derive(Clone, Encodable, Decodable, Debug)]
+#[derive(Copy, Clone, Encodable, Decodable, Debug)]
 pub struct ModSpans {
     /// `inner_span` covers the body of the module; for a file module, its the whole file.
     /// For an inline module, its the span inside the `{ ... }`, not including the curly braces.
     pub inner_span: Span,
+    pub inject_use_span: Span,
 }
 
 impl Default for ModSpans {
     fn default() -> ModSpans {
-        ModSpans { inner_span: Default::default() }
+        ModSpans { inner_span: Default::default(), inject_use_span: Default::default() }
     }
 }