Auto merge of #86190 - asquared31415:extern-main-86110-fix, r=varkor

Fix ICE when `main` is declared in an `extern` block

Changes in #84401 to implement `imported_main` changed how the crate entry point is found, and a declared `main` in an `extern` block was detected erroneously.  This was causing the ICE described in #86110.

This PR adds a check for this case and emits an error instead.  Previously a `main` declaration in an `extern` block was not detected as an entry point at all, so emitting an error shouldn't break anything that worked previously.  In 1.52.1 stable this is demonstrated, with a `` `main` function not found`` error.

Fixes #86110

compiler/rustc_middle/src/ty/mod.rs

@@ -128,7 +128,7 @@ pub struct ResolverOutputs {
     pub main_def: Option<MainDefinition>,
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug)]
 pub struct MainDefinition {
     pub res: Res<ast::NodeId>,
     pub is_import: bool,