Clang: Fix text format for completion details

The tooltip text right to the completion list item is prepared for rich
text (html) interpretation, but the QLabel the text will finally be
displayed with has no explicit text format set and thus defaults to auto
detection. The auto detection works fine for e.g. "<i>int optionalArg</
i>" but fails for "const Foo&amp;".

Task-number: QTCREATORBUG-15630
Change-Id: Ia58d65ee542730e4823c69150d452cdde98112f8
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>