Bump --enable-compiler-plugins Clang baseline?

[Noel Grandin]

> 
> On Wed, Nov 04, 2020 at 04:31:25PM +0100, Stephan Bergmann <sbergman at redhat.com> wrote:
>> So, what is the maximum Clang version that people would be comfortable with
>> here?
> 
>> lode already happens to provide a recipe to install Clang 9.0.1 on Linux. so
>> my suggestion---absent other constraints---would be to at least bump to
>> Clang 9, somewhat randomly.

I have 10.0.0-4 on Ubuntu