[NFC][AIX] Disable precompiled module file test on AIX

This patch follows the same reasoning as D114481. The PCH reader looks for `__clangast` section in the precompiled module file, which is not present in the file on AIX and not supported in XCOFF yet.

Reviewed By: daltenty

Differential Revision: https://reviews.llvm.org/D121709

Author: jakeegan

clang/test/ClangScanDeps/modules-no-undeclared-includes.c

@@ -1,3 +1,7 @@
+// Unsupported on AIX because we don't support the requisite "__clangast"
+// section in XCOFF yet.
+// UNSUPPORTED: aix
+
 // RUN: rm -rf %t && mkdir %t
 // RUN: split-file %s %t