These errors resulted from the improved parsing provided
by clang, which required rethinking which elements of the
QtConcurrent API should be visible to clangqdoc. The basic
problem is that clang must see declarations for all types
used by a type, or else it either gets lost and fails to
parse a type correctly, or it simply refuses to include
the type in its AST.

Change-Id: Iaa699c287e67d1288fcb2d83a9dbbaf07521d0cc
Reviewed-by: Martin Smith <martin.smith@qt.io>