[clang] Small adjustments for -fexperimental-library

Move -lc++experimental before -lc++abi (that was forgotten in the
original patch), and mark a test as UNSUPPORTED on AIX. I contacted
the owners of the AIX bot that failed because I was unable to reproduce
the issue locally.

clang/lib/Driver/ToolChains/Hexagon.cpp

@@ -614,10 +614,10 @@ void HexagonToolChain::AddCXXStdlibLibArgs(const ArgList &Args,
   switch (Type) {
   case ToolChain::CST_Libcxx:
     CmdArgs.push_back("-lc++");
-    CmdArgs.push_back("-lc++abi");
-    CmdArgs.push_back("-lunwind");
     if (Args.hasArg(options::OPT_fexperimental_library))
       CmdArgs.push_back("-lc++experimental");
+    CmdArgs.push_back("-lc++abi");
+    CmdArgs.push_back("-lunwind");
     break;
 
   case ToolChain::CST_Libstdcxx:

clang/test/Driver/experimental-library-flag.cpp

@@ -1,7 +1,10 @@
-// On Windows, -stdlib=libc++ is currently ignored, so -lc++experimental is not added.
-// Once -stdlib=libc++ works on Windows, this XFAIL can be removed.
+// On some platforms, -stdlib=libc++ is currently ignored, so -lc++experimental is not added.
+// Once -stdlib=libc++ works on those, this XFAIL can be removed.
 // XFAIL: windows, x86_64-scei-ps4, x86_64-sie-ps5
 
+// For some reason, this fails with a core dump on AIX. This needs to be investigated.
+// UNSUPPORTED: powerpc-ibm-aix
+
 // RUN: %clangxx -fexperimental-library -stdlib=libc++ -### %s 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-LIBCXX %s
 // RUN: %clangxx -fexperimental-library -stdlib=libstdc++ -### %s 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-LIBSTDCXX %s
 // RUN: %clangxx -fexperimental-library -stdlib=libc++ -nostdlib++ -### %s 2>&1 | FileCheck --check-prefixes=CHECK,CHECK-NOSTDLIB %s