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

[Teresa Johnson via llvm-commits]

================
@@ -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();
----------------
teresajohnson wrote:

For this change and at least one below, do we need to invoke fdStream() or can it simply continue to use FD which should have been initialized in the constructor?

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