Add a lint pass to check for while true { ... } loops

And suggest changing them to loop { ... }. Had to fix the few
remaining while true loops (in core::io). Closes #1962.

src/test/compile-fail/issue-1962.rs

@@ -0,0 +1,8 @@
+// compile-flags: -W err-while-true
+fn main() {
+  let mut i = 0;
+  while true  { //! ERROR Denote infinite loops with loop
+    i += 1;
+    if i == 5 { break; }
+  }
+}