[css] propose ids used in other selectors

### Issue Type Bug ### Description i have issues when i try to use suggestion for css files. for classes works well but when i'm using for id i have no suggestions try it with a file with like 500 lines of code ### VS Code Info VS Code version: Code 1.20.0 (c63189deaa8e620f650cc28792b8f5f3363f2c5b, 2018-02-07T17:09:39.780Z) OS version: Windows_NT x64 10.0.16299 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i5-5200U CPU @ 2.20GHz (4 x 2195)| |Memory (System)|3.91GB (0.52GB free)| |Process Argv|C:\Program Files\Microsoft VS Code\Code.exe| |Screen Reader|no| |VM|0%| </details><details><summary>Extensions (5)</summary> Extension|Author (truncated)|Version ---|---|--- rainbow-brackets|2gu|0.0.6 vscode-eslint|dba|1.4.5 vsc-material-theme|Equ|1.3.0 prettier-vscode|esb|1.1.3 LiveServer|rit|3.2.0 </details> Reproduces without extensions