[AIX] llvm-link support big archive.

Summary:
Use object::Archive::create so that the returned archive object has a dynamic type of either Archive or BigArchive.

Reviewers: James Henderson,Fangrui Song
Differential Revision: https://reviews.llvm.org/D124940

llvm/test/tools/llvm-link/archivell.ll

@@ -1,4 +1,3 @@
-# XFAIL: system-aix
 # RUN: llvm-ar cr %t.fg.a %S/Inputs/f.ll %S/Inputs/g.ll
 # RUN: not llvm-link %S/Inputs/h.ll %t.fg.a -o %t.linked.bc 2>&1 | FileCheck %s
 

llvm/tools/llvm-link/llvm-link.cpp

@@ -164,11 +164,16 @@ static std::unique_ptr<Module> loadArFile(const char *Argv0,
   if (Verbose)
     errs() << "Reading library archive file '" << ArchiveName
            << "' to memory\n";
-  Error Err = Error::success();
-  object::Archive Archive(*Buffer, Err);
-  ExitOnErr(std::move(Err));
+  Expected<std::unique_ptr<object::Archive>> ArchiveOrError =
+      object::Archive::create(Buffer->getMemBufferRef());
+  if (!ArchiveOrError)
+    ExitOnErr(ArchiveOrError.takeError());
+
+  std::unique_ptr<object::Archive> Archive = std::move(ArchiveOrError.get());
+
   Linker L(*Result);
-  for (const object::Archive::Child &C : Archive.children(Err)) {
+  Error Err = Error::success();
+  for (const object::Archive::Child &C : Archive->children(Err)) {
     Expected<StringRef> Ename = C.getName();
     if (Error E = Ename.takeError()) {
       errs() << Argv0 << ": ";