Space leak in persistentHieFileRule

https://github.com/haskell/haskell-language-server/blob/e0a464282db913b6c7e0f2ec4504d24e2bcfb4b1/ghcide/src/Development/IDE/Core/Rules.hs#L506-L513

`del` in line 512 keeps two large byte strings alive:
* source code of the .hs file on disk
* source code recorded in the .hie file

This is leaked for all the modules in the project with an entry in the hiedb and a `.hie` file (all in most circumstances).

The memory leak is not flushed until the `HieAstResult` is actually used and the `PositionMapping` is forced.