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/function-prologue-stepping-no-split-stack.rs

@@ -152,7 +152,7 @@ fn non_immediate_args(a: BigStruct, b: BigStruct) {
 
 #[no_split_stack]
 fn binding(a: i64, b: u64, c: f64) {
-    let x = 0;
+    let x = 0i;
 }
 
 #[no_split_stack]