[Flang] Add code owner for the Driver

Andrzej proposed the design and led the work for the Flang Driver
over the past couple of years. He is continuing to function as
the primary reviewer and gate-keeper of the Driver.

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D147573

Author: kiranchandramohan

flang/CODE_OWNERS.TXT

@@ -12,3 +12,7 @@ beautification by scripts. The fields are: name (N), email (E), web-address
 N: Steve Scalpone
 E: [email protected]
 D: Anything not covered by others
+
+N: Andrzej Warzynski
+E: [email protected]
+D: Driver