Core.Char -> char in a couple spots (#6126)

examples/interop/cpp/hello_world.carbon

@@ -20,13 +20,13 @@ fn Run() {
 
   // TODO: Requires Core.String API.
   // let message: str = "Hello world!\n\n";
-  // for (c: Core.Char in message) {
+  // for (c: char in message) {
   //   Cpp.putchar((c as u8) as i32);
   // }
 
-  let message: array(Core.Char, 13) =
+  let message: array(char, 13) =
       ('H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!', '\n');
-  for (c: Core.Char in message) {
+  for (c: char in message) {
     // TODO: u8 should probably have an implicit cast to i32.
     Cpp.putchar((c as u8) as i32);
   }

toolchain/lower/testdata/builtins/char.carbon

@@ -14,8 +14,8 @@
 
 library "[[@TEST_NAME]]";
 
-fn Example() -> Core.Char {
-  var c: Core.Char = 'a';
+fn Example() -> char {
+  var c: char = 'a';
   return c;
 }