Rollup merge of #52615 - davidtwco:add-to-gitignore, r=nikomatsakis,Mark-Simulacrum

Rollup merge of #52615 - davidtwco:add-to-gitignore, r=nikomatsakis,Mark-Simulacrum

Added new lines to .gitignore.

There are a handful of files that I often find in my local working directories that I never want to commit that aren't covered in the `.gitignore` file:

`/mir_dump`:
Default output location from `-Z mir-dump=all` for a specific test, I can't think of a reason why this should ever be commited.

`tags*`/`TAGS*`:
I use `vim-gutentags` which outputs `tags` and `tags.temp` which I don't want commited. I also collapsed the `TAGS`, `TAGS.vi`, `TAGS.emacs` into `TAGS*`.

`Session.vim`:
I use `vim-obsession` to save my current session in Vim, it outputs a `Session.vim` file, this also shouldn't be commited.