Addressed post-commit comment https://reviews.llvm.org/D122746#inline-1175831

Author: diggerlin

llvm/test/tools/llvm-ar/default-xcoff.test

@@ -7,4 +7,4 @@
 ; RUN: echo "test big archive" > %t.txt
 ; RUN: not llvm-ar cr %t.ar %t.txt 2>&1 | FileCheck %s
 
-; CHECK: unsupported big AIX write operation
+; CHECK: big archive writer operation on AIX not yet supported

llvm/tools/llvm-ar/llvm-ar.cpp

@@ -945,7 +945,7 @@ static void performWriteOperation(ArchiveOperation Operation,
       Kind = !NewMembers.empty() ? getKindFromMember(NewMembers.front())
                                  : getDefaultForHost();
     if (Kind == object::Archive::K_AIXBIG)
-      fail("unsupported big AIX write operation");
+      fail("big archive writer operation on AIX not yet supported");
     break;
   case GNU:
     Kind = object::Archive::K_GNU;