Move the test suite to the "as" form for casts. XFAIL a few tests for LLVM.

src/test/run-fail/str-overrun.rs

@@ -5,12 +5,12 @@
 fn main() {
   let str s = "hello";
   let int x = 0;
-  check (s.(x) == u8(0x68));
+  check (s.(x) == (0x68 as u8));
 
   // NB: at the moment a string always has a trailing NULL,
   // so the largest index value on the string above is 5, not
   // 4. Possibly change this.
 
   // Bounds-check failure.
-  check (s.(x + 6) == u8(0x0));
+  check (s.(x + 6) == (0x0 as u8));
 }