Fix ICE when main is declared in an extern block

src/test/ui/extern/extern-main-issue-86110.rs

@@ -0,0 +1,7 @@
+// missing and missing2 exist to make sure that the error only happens on a `main` declaration
+extern "C" {
+    fn missing();
+    fn main();
+    //~^ the `main` function cannot be declared in an `extern` block
+    fn missing2();
+}

src/test/ui/extern/extern-main-issue-86110.stderr

@@ -0,0 +1,8 @@
+error: the `main` function cannot be declared in an `extern` block
+  --> $DIR/extern-main-issue-86110.rs:4:5
+   |
+LL |     fn main();
+   |     ^^^^^^^^^^
+
+error: aborting due to previous error
+