FindClang.cmake: look also for LLVM 11

(cherry picked from commit 934e2944)
1 job for !191 with work/kossebau/supportllvm11 in 31 minutes and 19 seconds (queued for 24 minutes and 22 seconds)
detached
Status Job ID Name Coverage
  Build
passed #77298
linux kf5-qt5 qt5.12

00:31:19

45.0%