Move Int parsing to i32 for #543 (#700)

Note this doesn't support other sizes or types, it just errors on them.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/testdata/struct3.carbon

@@ -3,10 +3,10 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 struct Point {
-  var x: Int;
-  var y: Int;
+  var x: i32;
+  var y: i32;
 }
 
-fn main() -> Int {
+fn main() -> i32 {
   return Point(.x = 1, .y = 2).x - 1;
 }