Unignore closure-bang.

This test was ignored long ago in
https://github.com/rust-lang/rust/pull/20578/ when the syntax for
closures was changed.

The current status is that a closure with an explicit `!` return type
will trigger the `unreachable_code` lint which appears to be the
original intent of the test
(https://github.com/rust-lang/rust/pull/16836). A closure without a
return type won't trigger the lint since the `!` type isn't inferred
(AFAIK). This restores the test to its original form.

tests/ui/lint/dead-code/closure-bang.rs

@@ -1,9 +1,7 @@
-// ignore-test FIXME(#20574)
-
 #![deny(unreachable_code)]
 
 fn main() {
-    let x = || panic!();
+    let x = || -> ! { panic!() };
     x();
     println!("Foo bar"); //~ ERROR: unreachable statement
 }

tests/ui/lint/dead-code/closure-bang.stderr

@@ -0,0 +1,17 @@
+error: unreachable statement
+  --> $DIR/closure-bang.rs:6:5
+   |
+LL |     x();
+   |     --- any code following this expression is unreachable
+LL |     println!("Foo bar");
+   |     ^^^^^^^^^^^^^^^^^^^ unreachable statement
+   |
+note: the lint level is defined here
+  --> $DIR/closure-bang.rs:1:9
+   |
+LL | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+