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/pattern_variable_fail.carbon

@@ -2,8 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-fn main() -> Int {
+fn main() -> i32 {
   // error
-  x : Int;
+  x : i32;
   return 1;
 }