Add regression test for rust-lang#95829

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

Author: JohnTitor

src/test/ui/extern/issue-95829.rs

@@ -0,0 +1,10 @@
+// edition:2018
+
+extern {
+    async fn L() { //~ ERROR: incorrect function inside `extern` block
+        //~^ ERROR: functions in `extern` blocks cannot have qualifiers
+        async fn M() {}
+    }
+}
+
+fn main() {}

src/test/ui/extern/issue-95829.stderr

@@ -0,0 +1,32 @@
+error: incorrect function inside `extern` block
+  --> $DIR/issue-95829.rs:4:14
+   |
+LL |   extern {
+   |   ------ `extern` blocks define existing foreign functions and functions inside of them cannot have a body
+LL |       async fn L() {
+   |  ______________^___-
+   | |              |
+   | |              cannot have a body
+LL | |
+LL | |         async fn M() {}
+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: functions in `extern` blocks cannot have qualifiers
+  --> $DIR/issue-95829.rs:4:14
+   |
+LL | extern {
+   | ------ in this `extern` block
+LL |     async fn L() {
+   |              ^
+   |
+help: remove the qualifiers
+   |
+LL |     fn L() {
+   |     ~~
+
+error: aborting due to 2 previous errors
+