[Flang] Add code owner for the Driver

[kiranchandramohan]

Summary

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.

flang/CODE_OWNERS.TXT

 This file is a list of the people responsible for ensuring that patches for a
 particular part of Flang are reviewed, either by themself or by someone else.
 They are also the gatekeepers for their part of Flang, with the final word on
 what goes in or not.
 
 The list is sorted by surname and formatted to allow easy grepping and
 beautification by scripts. The fields are: name (N), email (E), web-address
 (W), PGP key ID and fingerprint (P), description (D), snail-mail address
 (S) and (I) IRC handle. Each entry should contain at least the (N), (E) and
 (D) fields.
 
 N: Steve Scalpone
 E: [e-mail]
 D: Anything not covered by others
+
+N: Andrzej Warzynski
+E: [e-mail]
+D: Driver