Add an infinite loop construct

Add a loop {} construct for infinite loops, and use it in test
cases. See #1906 for details.

src/test/run-pass/terminate-in-initializer.rs

@@ -4,7 +4,7 @@
 
 use std;
 
-fn test_break() { while true { let x: @int = break; } }
+fn test_break() { loop { let x: @int = break; } }
 
 fn test_cont() { let i = 0; while i < 1 { i += 1; let x: @int = cont; } }