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

[Teresa Johnson via llvm-commits]

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@@ -93,31 +93,39 @@ class BitstreamWriter {
 
   /// If the related file stream supports reading, seeking and writing, flush
   /// the buffer if its size is above a threshold.
-  void FlushToFile() {
-    if (!FS)
+  void FlushToFile(bool OnClosing = false) {
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teresajohnson wrote:

Suggest adding a brief comment about new parameter meaning/behavior.

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