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/debuginfo/generic-static-method-on-struct-and-enum.rs

@@ -61,8 +61,8 @@ impl Enum {
 }
 
 fn main() {
-    Struct::static_method(1, 2);
-    Enum::static_method(-3, 4.5, 5);
+    Struct::static_method(1i, 2i);
+    Enum::static_method(-3i, 4.5f64, 5i);
 }
 
 fn zzz() {()}