[NFC] [llvm-ar] Change use of empty to fix test on AIX

[qiucf]

Summary

This is a workaround for building LLVM/Clang using XL compiler on AIX.

Due to internal bug in the compiler, Filter here will be wrongly evaluated in later if statement. Several tests are failing:

LLVM :: Object/archive-extract.test
LLVM :: Object/check_binary_output.ll
LLVM :: tools/llvm-ar/full-path-option.test
LLVM :: tools/llvm-ar/count.test
LLVM :: tools/llvm-ar/print.test
LLVM :: tools/llvm-ar/extract.test
LLVM :: tools/llvm-ar/absolute-paths.test
LLVM :: tools/llvm-ar/path-names.test
LLVM :: tools/llvm-ar/response-utf8.test

We're investigating about the reason. Currently this fix will make it work properly.

[jsji]

Please add comments for the workaround, so that we can find and revert once problem in build compiler is fixed.

[jsji]

In D107331#2922166, @jsji wrote:

Please add comments for the workaround, so that we can find and revert once problem in build compiler is fixed.

Actually, now we have a fix in build compiler already, so we might not need this workaround any more.

[hubert.reinterpretcast]

In D107331#2922596, @jsji wrote:

Actually, now we have a fix in build compiler already, so we might not need this workaround any more.

We will need to deploy the fix to the external-facing buildbots .

[qiucf]

Thanks. Now let's abandon this.

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

static bool shouldCreateArchive(ArchiveOperation Op) {
   llvm_unreachable("Missing entry in covered switch.");
 }
 
 static void performReadOperation(ArchiveOperation Operation,
                                  object::Archive *OldArchive) {
   if (Operation == Extract && OldArchive->isThin())
     fail("extracting from a thin archive is not supported");
 
-  bool Filter = !Members.empty();
+  bool Filter = Members.size() != 0;
   StringMap<int> MemberCount;
   {
     Error Err = Error::success();
     for (auto &C : OldArchive->children(Err)) {
       Expected<StringRef> NameOrErr = C.getName();
       failIfError(NameOrErr.takeError());
       StringRef Name = NameOrErr.get();