[lldb][NFC] Refactor Fix-It filter for warnings

LLDB only automatically applies Fix-Its from errors, but not from warnings.

Currently we only store Fix-Its from errors and then later apply all Fix-Its
we stored. This moves the filter to the application phase, so that we now
store *all* Fix-Its but only apply Fix-Its from errors later on.

This is NFC preparation for an upcoming patch.