Installer-packaged "lupdate -clang-parser" fails

lupdate's clang parser needs CLANG_RESOURCE_DIR which contains header
files that are read when building the AST. Qt doesn't ship clang
headers nor libraries. lupdate and qdoc are linking libclang
statically.

Qt Creator, on the other hand, ships libclang, including
CLANG_RESOURCE_DIR. One idea was to locate and use that directory. However, the
versions of libclang for Qt and Qt Creator differ. Especially when
using an older Qt (6.0.x built with libclang 10) with a current Qt
Creator built with libclang 11.

All this points to: we need to ship libclang with Qt.

That shipped libclang version could be built dynamically and shared
between qdoc and lupdate.