Fix an overflow issue at BackpatchWord

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.

Reviewed-by: tejohnson, vitalybuka

Differential Revision: https://reviews.llvm.org/D86645

llvm/include/llvm/Bitstream/BitstreamWriter.h

@@ -103,7 +103,7 @@ class BitstreamWriter {
   /// 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");