[llvm] Unittests and usability for BitstreamWriter incremental flushing (PR #92983)

[Mircea Trofin via llvm-commits]

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@@ -141,16 +209,16 @@ class BitstreamWriter {
     if (ByteNo >= NumOfFlushedBytes) {
       assert((!endian::readAtBitAlignment<uint8_t, llvm::endianness::little,
                                           unaligned>(
-                 &Out[ByteNo - NumOfFlushedBytes], StartBit)) &&
+                 &Buffer[ByteNo - NumOfFlushedBytes], StartBit)) &&
              "Expected to be patching over 0-value placeholders");
       endian::writeAtBitAlignment<uint8_t, llvm::endianness::little, unaligned>(
-          &Out[ByteNo - NumOfFlushedBytes], NewByte, StartBit);
+          &Buffer[ByteNo - NumOfFlushedBytes], NewByte, StartBit);
       return;
     }
 
     // If the byte offset to backpatch is flushed, use seek to backfill data.
     // First, save the file position to restore later.
-    uint64_t CurPos = FS->tell();
+    uint64_t CurPos = fdStream()->tell();
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mtrofin wrote:

we could use FD, but we should only be here if fdStream() != nullptr. The reason that happens is not very apparent, though: `GetNumOfFlushedBytes` returns 0 in that case, which makes the if-check above always succeed. Added an assert and a comment.

https://github.com/llvm/llvm-project/pull/92983