Use Cell instead of Vec in const-generics test

As `Cell` won't receive an allocator parameter, it is used. Otherwise a `#![feature(allocator_api)]` could have been added, but for the purpose of this test, changing the type is more clear.

Author: TimDiekmann

src/test/ui/const-generics/invalid-constant-in-args.rs

@@ -1,4 +1,6 @@
+use std::cell::Cell;
+
 fn main() {
-    let _: Vec<&str, "a"> = Vec::new();
+    let _: Cell<&str, "a"> = Cell::new("");
     //~^ ERROR wrong number of generic arguments
 }

src/test/ui/const-generics/invalid-constant-in-args.stderr

@@ -1,8 +1,8 @@
 error[E0107]: wrong number of generic arguments: expected 1, found 2
-  --> $DIR/invalid-constant-in-args.rs:2:22
+  --> $DIR/invalid-constant-in-args.rs:4:23
    |
-LL |     let _: Vec<&str, "a"> = Vec::new();
-   |                      ^^^ unexpected const argument
+LL |     let _: Cell<&str, "a"> = Cell::new("");
+   |                       ^^^ unexpected const argument
 
 error: aborting due to previous error