coverage: Set up MC/DC bitmaps without additional unsafe code

Because this now always takes place at the start of the function, we can just
use the normal `alloca` method and then initialize each bitmap immediately.

This patch also moves bitmap setup out of the `mcdc_parameters` method, because
there is no longer any particular reason for it to be there.

compiler/rustc_abi/src/lib.rs

@@ -21,6 +21,8 @@ use rustc_macros::{Decodable_Generic, Encodable_Generic};
 use std::iter::Step;
 
 mod layout;
+#[cfg(test)]
+mod tests;
 
 pub use layout::LayoutCalculator;