[NFC][tests][AIX] XFAIL test for lack of visibility support

With the addition of `__attribute__((visibility("hidden")))` to the test, the test fails because AIX's current default behaviour is to ignore hidden visibility, so the expected error is not seen. This patch marks the test `XFAIL` on AIX for now.

Reviewed By: cebowleratibm

Differential Revision: https://reviews.llvm.org/D122519

clang/test/OpenMP/target_update_messages.cpp

@@ -1,3 +1,6 @@
+// Visibility hidden is not currently implemented on AIX.
+// XFAIL: aix
+
 // RUN: %clang_cc1 -verify=expected,lt50,lt51 -fopenmp -fopenmp-version=45 -ferror-limit 100 -o - -std=c++11 %s -Wuninitialized
 // RUN: %clang_cc1 -verify=expected,ge50,lt51 -fopenmp -fopenmp-version=50 -ferror-limit 100 -o - -std=c++11 %s -Wuninitialized
 // RUN: %clang_cc1 -verify=expected,ge50,ge51 -fopenmp -fopenmp-version=51 -ferror-limit 100 -o - -std=c++11 %s -Wuninitialized