[Bitcode] Get rid of compiler message (llvm#113428)

Insert explicit cast from an enumerator to unsigned int, because some
compilers issue a warning on signed vs unsigned comparison, see:
llvm#110805 (comment).

Author: spavloff

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -798,7 +798,7 @@ class BitcodeReader : public BitcodeReaderBase, public GVMaterializer {
     if (Slot == Record.size())
       return true;
     unsigned ValID = Record[Slot++];
-    if (ValID != bitc::OB_METADATA) {
+    if (ValID != static_cast<unsigned>(bitc::OB_METADATA)) {
       unsigned TypeId;
       return getValueTypePair(Record, --Slot, InstNum, ResVal, TypeId,
                               ConstExprInsertBB);