Revert "Remove rule requiring non-nil block-style statements to be semi-terminated"

This reverts commit 0f5eaef5fb.

src/libsyntax/ast.rs

@@ -243,7 +243,12 @@ type stmt = spanned<stmt_>;
 #[auto_serialize]
 enum stmt_ {
     stmt_decl(@decl, node_id),
+
+    // expr without trailing semi-colon (must have unit type):
     stmt_expr(@expr, node_id),
+
+    // expr with trailing semi-colon (may have any type):
+    stmt_semi(@expr, node_id),
 }
 
 #[auto_serialize]