Bump --enable-compiler-plugins Clang baseline?

[Stephan Bergmann]

Since 
<https://git.libreoffice.org/core/+/f23aa1a51cb1beea4ebe3a61ba0c9b3abd844fd9%5E!/> 
"Bump compiler plugins Clang baseline to 5.0.2" from about two years 
ago, the baseline for (Linux) --enable-compiler-plugins builds is Clang 
5.0.2.

Wasting time yesterday with 
<https://gerrit.libreoffice.org/c/core/+/105234/1..4>, tracking down why 
a loplugin:toolslong false positive started to appear with old Clang, I 
wondered whether we could bump once again.  The benefit would be getting 
rid of various #if CLANG_VERSION cruft across compilerplugins/clang/, 
and potentially avoiding wasting time on similar issues in the future. 
(Plus, we could revisit 
<https://lists.freedesktop.org/archives/libreoffice/2019-November/083780.html> 
"On using C++17 for compilerplugins (not possible for now)".)

The question is what the maximum Clang version would be that we could 
bump to as a baseline for (Linux) --enable-compiler-plugins builds. 
(For (implicit) --disable-compiler-plugins builds, baselines can stay as 
they are for now.  And on macOS and Windows, I think I am the only one 
using --enable-compiler-plugins, and I'm using Clang trunk there anyway.)

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.