Add test case for wasm non-clash.

src/test/ui/lint/clashing-extern-fn-wasm.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![crate_type = "lib"]
+
+#[cfg(target_arch = "wasm32")]
+mod wasm_non_clash {
+    mod a {
+        #[link(wasm_import_module = "a")]
+        extern "C" {
+            pub fn foo();
+        }
+    }
+
+    mod b {
+        #[link(wasm_import_module = "b")]
+        extern "C" {
+            pub fn foo() -> usize;
+            // #79581: These declarations shouldn't clash because foreign fn names are mangled
+            // on wasm32.
+        }
+    }
+}