Add rustfmt::skip to some files

Five of the files being skipped here are because rustfmt is buggy (see
the error messages below). The others have clearly preferable manual
formatting.

    error[internal]: left behind trailing whitespace
      --> tests/fail/validity/transmute_through_ptr.rs:18:18:1
       |
    18 |
       | ^^^^
       |

    warning: rustfmt has failed to format. See previous 1 errors.

    error[internal]: left behind trailing whitespace
      --> tests/fail/stacked_borrows/illegal_read2.rs:10:10:1
       |
    10 |
       | ^^^^
       |

    warning: rustfmt has failed to format. See previous 1 errors.

    error[internal]: left behind trailing whitespace
      --> tests/fail/stacked_borrows/illegal_read5.rs:15:15:1
       |
    15 |
       | ^^^^
       |

    warning: rustfmt has failed to format. See previous 1 errors.

    error[internal]: left behind trailing whitespace
      --> tests/fail/stacked_borrows/illegal_read1.rs:10:10:1
       |
    10 |
       | ^^^^
       |

    warning: rustfmt has failed to format. See previous 1 errors.

    error[internal]: left behind trailing whitespace
     --> tests/fail/erroneous_const2.rs:9:9:1
      |
    9 |
      | ^^^^
      |

    warning: rustfmt has failed to format. See previous 1 errors.

tests/fail/erroneous_const2.rs

@@ -4,6 +4,7 @@
 //~^ERROR any use of this value
 //~|WARN previously accepted
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     println!("{}", FOO); //~ERROR post-monomorphization error
     //~|ERROR evaluation of constant value failed

tests/fail/stacked_borrows/illegal_read1.rs

@@ -1,6 +1,7 @@
 // A callee may not read the destination of our `&mut` without
 // us noticing.
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let mut x = 15;
     let xraw = &mut x as *mut _;

tests/fail/stacked_borrows/illegal_read2.rs

@@ -1,6 +1,7 @@
 // A callee may not read the destination of our `&mut` without
 // us noticing.
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let mut x = 15;
     let xraw = &mut x as *mut _;

tests/fail/stacked_borrows/illegal_read5.rs

@@ -5,6 +5,7 @@
 use std::cell::RefCell;
 use std::{mem, ptr};
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let rc = RefCell::new(0);
     let mut refmut = rc.borrow_mut();

tests/fail/validity/transmute_through_ptr.rs

@@ -7,6 +7,7 @@ fn evil(x: &mut Bool) {
     unsafe { *x = 44; } // out-of-bounds enum tag
 }
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let mut x = Bool::True;
     evil(&mut x);

tests/pass/enums.rs

@@ -64,6 +64,7 @@ pub enum E1 {
     // from the `bool` field of `V1`), overflowing for variants with large enough
     // indices (`V3` and `V4`), causing them to be interpreted as other variants.
     #[allow(dead_code)]
+    #[rustfmt::skip] // rustfmt prefers every variant on its own line
     pub enum E2<X> {
         V1 { f: bool },