QDoc is build against libclang 4.0 while it should be using at least 6.0

It seems that on Windows libclang 6.0 is needed. Our CI is using version 4.0

{quote}

nierob@43 ~/d/qt5> git status
On branch 5.11
Your branch is up to date with 'origin/5.11'.
....
nierob@43 ~/d/qt5> cat coin/provisioning/common/shared/sw_versions.txt
libclang_version="4.0"

{quote}