When the pipeline just copies the file, prepend a LINE pragma
For example, "ghc -E Foo.hs -o Foo.bar" just copies Foo.hs to Foo.bar. This patch adds a LINE pragma to the beginning of Foo.bar so that further processing can track the location of the original file. The motiviation for this is bug #1044. When generating Haddock docs, we preprocess the .hs to a .raw-hs, sometimes this doesn't involve any actual preprocessing and in those cases we lose track of the original filename.