add regression test for rust-lang#85763

Author: lolbinarycat

tests/rustdoc/heterogeneous-concat.rs

@@ -0,0 +1,11 @@
+// regression test for https://github.com/rust-lang/rust/issues/85763
+
+#![crate_name = "foo"]
+
+//@ has foo/index.html '//main' 'Some text that should be concatenated.'
+#[doc = " Some text"]
+#[doc = r" that should"]
+/// be concatenated.
+pub fn main() {
+    println!("Hello, world!");
+}