Remove the special case for i32. (#4543)

For the few remaining uses of the builtin `i32` type, manually build an
`IntType(Signed, 32)` value instead. These are:

- The return type of `Run`.
- The type that int literals in an `if` expression are converted into.
- The type of an array index expression.

We should consider converting those three cases away from `i32` over
time.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/handle_function.cpp

@@ -300,8 +300,7 @@ static auto BuildFunctionDecl(Context& context,
         !function_info.param_patterns_id.is_valid() ||
         !context.inst_blocks().Get(function_info.param_patterns_id).empty() ||
         (return_type_id.is_valid() &&
-         return_type_id !=
-             context.GetBuiltinType(SemIR::BuiltinInstKind::IntType) &&
+         return_type_id != context.GetInt32Type() &&
          return_type_id != context.GetTupleType({}))) {
       CARBON_DIAGNOSTIC(InvalidMainRunSignature, Error,
                         "invalid signature for `Main.Run` function; expected "