feat(selfhost-sema): check annotation vs initializer type in let/var

_test_error_recovery/src/Main.bux

@@ -1,5 +1,6 @@
 func Main() -> int {
-    let bad: int = "a" + 1;
+    let x: int = "hello";
+    let y: bool = 42;
     let z: int = undefined_variable;
     return 0;
 }