[clang] [libclang] Document CXUnsavedFile::Length width and fix test narrowing (PR #228405)

[via cfe-commits]

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@@ -118,6 +118,11 @@ struct CXUnsavedFile {
 
   /**
    * The length of the unsaved contents of this buffer.
+   *
+   * This is intentionally \c unsigned \c long, not a fixed-width type, to
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UebelAndre wrote:

Updated! I added the ABI-stability rationale to the commit message instead

https://github.com/llvm/llvm-project/pull/228405