qdoc: Clear the map of HTML anchors for each generated page

To avoid conflicting anchors within an HTML page, QDoc keeps
the used anchors in a map, and appends 'x' to new anchors if
it was already used. However, this map was never cleared, so
anchors on one page affected the ones generated for another
page.

This commit clears the map for each page when generating the
page header.

Change-Id: Id30baced46917bb4d48cf58dde047f4fbcdf74cf
Task-number: QTBUG-46153
Reviewed-by: Martin Smith <martin.smith@digia.com>

src/tools/qdoc/htmlgenerator.cpp

@@ -1986,6 +1986,7 @@ void HtmlGenerator::generateHeader(const QString& title,
     out() << QString(postPostHeader).replace("\\" + COMMAND_VERSION, qdb_->version());
 
     navigationLinks.clear();
+    refMap.clear();
 
     if (node && !node->links().empty()) {
         QPair<QString,QString> linkPair;