...to free some memory.

The translation units of the 7 most recently used documents ("hot
documents", tracked by document visibility) are kept in memory.
Translation units of other documents are suspended and will be resumed
once they become visible again.

The resumption of a translation unit needs the same time as reparse
(since it is a reparse effectively).

The number of hot documents can be modified by the run time environment
variable QTC_CLANG_HOT_DOCUMENTS=N. Visible documents are always hot.

Task-number: QTCREATORBUG-11640
Change-Id: I68ecd2b1373e303372300203e42d90f65a4b39b3
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>