Handle borrows of unions in NLL

Author: KiChjang

text/2094-nll.md

@@ -1563,6 +1563,8 @@ follows:
 
 - any loans whose region does not include P are killed;
 - if this is a borrow statement, the corresponding loan is generated;
+  - if the borrow contains a union field, loans are generated for all
+    prefixes of the union field;
 - if this is an assignment `lv = <rvalue>`, then any loan for some path P
   of which `lv` is a prefix is killed.