Auto merge of #32944 - alexcrichton:add-to-gitignore, r=brson

Add /obj/ to .gitignore

This is the build directory our buildbots use, and right now the bots are
running `git clean -f -f -d` to remove all untracked files between runs and this
is accidentally deleting `obj`, so we're building LLVM a lot.

Hopefully this keeps the bots caching `obj` so we can clean it out manually and
leave LLVM around.