Create a build job for building libclang specifically for qdoc

For Qt 5.11 we hacked the build scripts and job on Jenkins that we use for creating LLMV/Clang builds for Qt Creator, as a quick-and-dirty fix for building qdoc.

While it was ok-ish to do that for Qt 5.11, it is not a good situation. We want some libclang version for use by qdoc, without it being affected by whatever we decide to do for Qt Creator. E.g. for Qt Creator we want to upgrade the LLVM/Clang version, we apply some patches, we build the Clazy&co. into libclang, do profile optimization etc, which is all not wanted for qdoc.

Also, the Qt Creator LLVM/Clang is currently built on Jenkins, which the people caring about the Qt build infrastructure do not really know about, and they shouldn't need to.

The libclang for qdoc should instead be built on infrastructure used for other things in Qt, so that it can also be maintained by the people who are responsible for it.