[xml] Highlight problem

[tradzero]

Binaryify/OneDark-Pro#136

• VSCode Version:1.15.0 X64
• OS Version:Windows 10 15063

Steps to Reproduce:

1.comment a line

Reproduces without extensions: Yes

file code is here

[aeschli]

This is not valid XML, as confirmed by http://www.xmlvalidation.com/.
The comment can not be inside attributes.