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/lexical-scope-in-unconditional-loop.rs

@@ -84,7 +84,7 @@
 
 fn main() {
 
-    let mut x = 0;
+    let mut x = 0i;
 
     loop {
         if x >= 2 {
@@ -108,7 +108,7 @@ fn main() {
             zzz();
             sentinel();
 
-            let x = -987;
+            let x = -987i;
 
             zzz();
             sentinel();