Debounce work done progress that shows the index update

[ahoppen]

We currently create a work done progress in the editor as soon as a file starts to be indexed. While this is great to see exactly what’s going on indexing-wise, it might be too noisy for users if Indexing: 0/1 shows up for fractions of a section whenever a file is saved. We should consider debouncing its display.

[ahoppen]

Synced to Apple’s issue tracker as rdar://128071328