Revert "[AIX][BigArchive] Treat the archive is empty if the first child member offset is zero"

This reverts commit b26fe2a.

Reason: Broke the ASan build bots. See more info in the original
phabricator review: https://reviews.llvm.org/D138986

Author: hctim

llvm/include/llvm/Object/Archive.h

@@ -413,7 +413,9 @@ class BigArchive : public Archive {
   BigArchive(MemoryBufferRef Source, Error &Err);
   uint64_t getFirstChildOffset() const override { return FirstChildOffset; }
   uint64_t getLastChildOffset() const { return LastChildOffset; }
-  bool isEmpty() const override { return getFirstChildOffset() == 0; }
+  bool isEmpty() const override {
+    return Data.getBufferSize() == sizeof(FixLenHdr);
+  };
 };
 
 } // end namespace object

llvm/test/Object/archive-big-malformed-first-member.test

@@ -1,3 +1,3 @@
 # Test reading an empty archive with free list in it.
-# RUN: llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1 \
-# RUN:   | not grep 'truncated or malformed archive'
+# RUN: not llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1 \
+# RUN:   | grep 'truncated or malformed archive'