librustc: Remove the fallback to int for integers and f64 for

floating point numbers for real.

This will break code that looks like:

    let mut x = 0;
    while ... {
        x += 1;
    }
    println!("{}", x);

Change that code to:

    let mut x = 0i;
    while ... {
        x += 1;
    }
    println!("{}", x);

Closes #15201.

[breaking-change]

src/test/compile-fail/for-loop-refutable-pattern-error-message.rs

@@ -11,6 +11,6 @@
 
 fn main() {
     for
-        &1 //~ ERROR refutable pattern in `for` loop binding
-        in [1].iter() {}
+        &1i //~ ERROR refutable pattern in `for` loop binding
+        in [1i].iter() {}
 }