[test][CAS] Fix unused variable warning in unittest (#159594)

Fix unused variable warning blocking AIX bot.

Author: cachemeifyoucan

llvm/unittests/CAS/ProgramTest.cpp

@@ -26,6 +26,8 @@ extern char **environ;
 using namespace llvm;
 using namespace llvm::cas;
 
+#if LLVM_ENABLE_ONDISK_CAS
+
 extern const char *TestMainArgv0;
 static char ProgramID = 0;
 
@@ -81,8 +83,6 @@ class CASProgramTest : public testing::Test {
   ArrayRef<StringRef> getEnviron() const { return EnvTable; }
 };
 
-#if LLVM_ENABLE_ONDISK_CAS
-
 static Error emptyConstructor(MappedFileRegionArena &) {
   return Error::success();
 }