Add const_for test

Author: jonas-schievink

src/test/ui/consts/const-for.rs

@@ -0,0 +1,10 @@
+#![feature(const_for)]
+#![feature(const_mut_refs)]
+
+const _: () = {
+    for _ in 0..5 {}
+    //~^ error: calls in constants are limited to
+    //~| error: calls in constants are limited to
+};
+
+fn main() {}

src/test/ui/consts/const-for.stderr

@@ -0,0 +1,15 @@
+error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/const-for.rs:5:14
+   |
+LL |     for _ in 0..5 {}
+   |              ^^^^
+
+error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/const-for.rs:5:14
+   |
+LL |     for _ in 0..5 {}
+   |              ^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0015`.