Do not ICE when lowering invalid extern fn with bodies

Fix #75283.

src/test/ui/issues/issue-75283.rs

@@ -0,0 +1,6 @@
+extern "C" {
+    fn lol() { //~ ERROR incorrect function inside `extern` block
+        println!("");
+    }
+}
+fn main() {}

src/test/ui/issues/issue-75283.stderr

@@ -0,0 +1,18 @@
+error: incorrect function inside `extern` block
+  --> $DIR/issue-75283.rs:2:8
+   |
+LL |   extern "C" {
+   |   ---------- `extern` blocks define existing foreign functions and functions inside of them cannot have a body
+LL |       fn lol() {
+   |  ________^^^___-
+   | |        |
+   | |        cannot have a body
+LL | |         println!("");
+LL | |     }
+   | |_____- help: remove the invalid body: `;`
+   |
+   = help: you might have meant to write a function accessible through FFI, which can be done by writing `extern fn` outside of the `extern` block
+   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
+
+error: aborting due to previous error
+