[READY] Restore cursor position after omnifunc calls

An omnifunc may move the cursor position even on the first call, e.g. [`completeR` from the `Nvim-R` plugin](https://github.com/jalvesaq/Nvim-R/blob/0db20893d34654aefe2ea1027d815159556330a7/R/common_global.vim#L3383). The cursor position is not restored between the two calls because it's unnecessary with the changes from PR #3112.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/valloric/youcompleteme/3117)
<!-- Reviewable:end -->