[analyzer] Use sed from the ToolBox on AIX (NFC) (#161242)

The change in commit 30402c7 breaks the tests on AIX. This patch 
is to change to use the `sed` from AIX Toolbox instead of the default 
one which does not support `-r` and `-E`.

Author: kkwli

clang/test/Analysis/diagnostics/Inputs/expected-sarif/sarif-multi-file-diagnostics.c.sarif

@@ -141,4 +141,4 @@
     }
   ],
   "version": "[SARIF version]"
-}
+}

clang/test/Analysis/lit.local.cfg

@@ -17,11 +17,13 @@ config.substitutions.append(
     )
 )
 
+sed_cmd = "/opt/freeware/bin/sed" if "system-aix" in config.available_features else "sed"
+
 # Filtering command for testing SARIF output against reference output.
 config.substitutions.append(
     (
         "%normalize_sarif",
-        "sed -r '%s;%s;%s;%s'"
+        f"{sed_cmd} -r '%s;%s;%s;%s'"
         % (
             # Replace version strings that are likely to change.
             r's/"version": ".* version .*"/"version": "[clang version]"/',