Semanticdb clear dir

[crt-31]

Description

This change clears out and deletes the _semanticdb folder before compiling. This is similar to how ScalacWorker was already clearing out the _classes directory.

Motivation

I ran into a bug where, when you delete a source *.scala file, it's associated semanticdb file remained and was read-only which caused permission errors when the compiler tried to clear the semanticdb output directory.