Diagnostic problems are not shown on typing first character and after deleting all the characters causing the error

[nayankankariya]

Steps :

1. Set "validateOnType" preference as true in brackets.json file. and save the file
2. Open any php file
3. type f

Result : no error shown in diagnostic problems

Expected : errors should be shown.
4) type fo

Result : errors start showing
5) remove all characters till f by hitting backspace

Result: even after removal of all characters causing the error, the ; missing error is still shown

Expected: error should get removed from the list

[nayankankariya]

Fix is checked in as part of PR #14685