[Just a note to myself as I always need to look up what to download and install on Visual Studio 2017 to allow me to work on RDLC layouts for my reports.]
When opening the layout of a printable report from C/SIDE report designer Visual Studio 2017 will be opened. When you have a new installation of VS 2017, the layout will however be shown in xml format only. In previous versions of Visual Studio the issue was solved by installing SQL Data Tools Trying the same for VS 2017 will not help you.
Thanx to Natalie Karolak and Steven Renders for pointing to the solution. According to the System Requirements for NAV 2018 you need to install Microsoft Rdlc Report Designer for Visual Studio.