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

[Aaron Ballman via cfe-commits]

================
@@ -118,6 +118,9 @@ struct CXUnsavedFile {
 
   /**
    * The length of the unsaved contents of this buffer.
+   *
+   * Note that this is an \c unsigned \c long, which has a platform-dependent
+   * width. Do not assume it is a 64-bit integer.
----------------
AaronBallman wrote:

Yeah, I think we should pull the comments from here. The original issue is not really a bug, though it could perhaps be a feature request (whether it's warranted is another matter, but that can be debated on the issue).

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