* 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
ae119a9a8d