Disallow to optimise out constants in OOB tests

Apparently our constant errors are only reported if there’s a use in MIR for that constant. This
means that in these tests `let _ = B` is seen as a no-op use and thus is optimised out. Explicitly
drop() the B to make sure it is not optimised (yet, until the inliner happens).

NB: the `let _ = B` statement discussed here has no side effects (such as Drop implementation).

Author: nagisa

src/test/compile-fail/array_const_index-0.rs

@@ -14,5 +14,5 @@ const B: i32 = (&A)[1];
 //~| index out of bounds: the len is 0 but the index is 1
 
 fn main() {
-    let _ = B;
+    drop(B);
 }

src/test/compile-fail/array_const_index-1.rs

@@ -14,5 +14,5 @@ const B: i32 = A[1];
 //~| index out of bounds: the len is 0 but the index is 1
 
 fn main() {
-    let _ = B;
+    drop(B);
 }

src/test/compile-fail/const-array-oob.rs

@@ -20,5 +20,5 @@ const BLUB: [u32; FOO[4]] = [5, 6];
 //~| index out of bounds: the len is 3 but the index is 4
 
 fn main() {
-    let _ = BAR;
+    drop(BAR);
 }

src/test/compile-fail/const-slice-oob.rs

@@ -14,5 +14,5 @@ const BAR: u32 = FOO[5];
 //~| index out of bounds: the len is 3 but the index is 5
 
 fn main() {
-    let _ = BAR;
+    drop(BAR);
 }