Fix an overflow issue at BackpatchWord

[stephan.yichao.zhao]

Summary

This happens when generating a huge file by LTO, for example, with -gmlt.
When BitNo is > 2^35, ByteNo is overflowed, and an incorrect output offset is overwritten.
This generates ill-formed bitcodes.

[tejohnson]

lgtm

llvm/include/llvm/Bitstream/BitstreamWriter.h

public:
   //===--------------------------------------------------------------------===//
   // Basic Primitives for emitting bits to the stream.
   //===--------------------------------------------------------------------===//
 
   /// Backpatch a 32-bit word in the output at the given bit offset
   /// with the specified value.
   void BackpatchWord(uint64_t BitNo, unsigned NewWord) {
     using namespace llvm::support;
-    unsigned ByteNo = BitNo / 8;
+    uint64_t ByteNo = BitNo / 8;
     assert((!endian::readAtBitAlignment<uint32_t, little, unaligned>(
                &Out[ByteNo], BitNo & 7)) &&
            "Expected to be patching over 0-value placeholders");
     endian::writeAtBitAlignment<uint32_t, little, unaligned>(
         &Out[ByteNo], NewWord, BitNo & 7);
   }
 
   void BackpatchWord64(uint64_t BitNo, uint64_t Val) {