qdoc: Sanitize anchors in URLs for functions

When QDoc constructs the full path to a documentation node, it
must construct a clean anchor reference. Intra-page links use
a different code path, so the links e.g. from Member Functions
list to their detailed descriptions always work, but using the
link command to link to functions with certain characters (such
as 'operator==') failed because the node name was used as-is and
not sanitized ('operator-eq-eq').

This change moves HtmlGenerator::cleanRef() static function to
its parent class, Generator, and takes it into use in
Generator::fullDocumentLocation().

Change-Id: Ic939ffa3ae0f4495ef2a30eff0d4a1de65ea3e8f
Task-number: QTBUG-45629
Reviewed-by: Martin Smith <martin.smith@digia.com>