FunctionDeclDefLink::changes is called from
CPPEditorWidget::updateFunctionDeclDefLinkNow() where it receives the
snapshot by reference from the semantic info. The semantic info is
recalculated in a/the future, so it might change the snapshot or its
documents while it's still being used by the decl/def link finder.

So, this patch takes a copy of all relevant semantic info (snapshot and
document) before starting to calculate the changes.

Change-Id: I78244a4ca8149233403b3c35ee05a2d4ed4b2770
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>