CLI: fix stdin dumping behavior

 * no need to move `tmpFilePath` around
 * no need for calculating max length of `FileExt` tag name
 * provide a canonical file extension name for `FileExt` so that, e.g.
   the file will be named `stdin.S` instead of
   `stdin.assembly_with_cpp`.
 * move temp file cleanup to a function to reduce defer bloat in a large
   function.
 * fix bug caused by mixing relative and absolute paths in the cleanup
   logic.
 * remove commented out test and dead code

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