NFC: add code owners for C++ interop

This teaches GitHub to automatically add the three of us as reviewers to patches that alter the source files specific to C++ interop.

This doesn't mean that all of the people added as reviewers must necessarily review a patch, is just makes it more straightforward for folks who don't have commit access to request reviews for their patches.

Author: egorzhdan

.github/CODEOWNERS

@@ -6,3 +6,8 @@
 include/swift/Parse @ahoppen @bnbarham @CodaFi @DougGregor @rintaro
 lib/Parse           @ahoppen @bnbarham @CodaFi @DougGregor @rintaro
 test/Parse          @ahoppen @bnbarham @CodaFi @DougGregor @rintaro
+
+# C++ Interop
+stdlib/public/Cxx       @zoecarver @hyp @egorzhdan
+test/Interop/Cxx        @zoecarver @hyp @egorzhdan
+test/Interop/SwiftToCxx @zoecarver @hyp @egorzhdan