[NFC][AIX]Disable new pcm tests on AIX

[stevewan]

Summary

Same as D114481, the PCH reader looks for a __clangast section in the precompiled module file, which isn't present on AIX, and we don't support writing this custom section in XCOFF yet.

[Jake-Egan]

LGTM.

[daltenty]

LGTM, thanks!

[sfertile]

Thanks Steven. LGTM.

clang/test/ClangScanDeps/modules-symlink.c

 // RUN: rm -rf %t
 // RUN: split-file %s %t
-// UNSUPPORTED: system-windows
+// Unsupported on AIX because we don't support the requisite "__clangast"
+// section in XCOFF yet.
+// UNSUPPORTED: system-windows, aix
 
 //--- cdb_pch.json
 [
   {
     "directory": "DIR",
     "command": "clang -x c-header DIR/pch.h -fmodules -gmodules -fimplicit-module-maps -fmodules-cache-path=DIR/cache -o DIR/pch.h.gch",
     "file": "DIR/pch.h"
   }