Silence a -Wsign-compare warning; NFC.

llvm-svn: 246646
diff --git a/clang/lib/Serialization/MultiOnDiskHashTable.h b/clang/lib/Serialization/MultiOnDiskHashTable.h
index 3ef8103..04dea83 100644
--- a/clang/lib/Serialization/MultiOnDiskHashTable.h
+++ b/clang/lib/Serialization/MultiOnDiskHashTable.h
@@ -215,7 +215,7 @@
     if (!PendingOverrides.empty())
       removeOverriddenTables();
 
-    if (Tables.size() > Info::MaxTables)
+    if (Tables.size() > static_cast<unsigned>(Info::MaxTables))
       condense();
 
     internal_key_type Key = Info::GetInternalKey(EKey);