ClangTools: Fix clang-tidy documentation URLs

The model for the clang-tidy checks absurdly uses a FilePath-based base
class, and after cd919e6f61 started
producing wrong URLs. Work around that.

Change-Id: I8ef4f678c3abf991ada1a2955baa3d1ec13a195b
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangtools/diagnosticconfigswidget.cpp

@@ -263,7 +263,8 @@ static void buildTree(ProjectExplorer::Tree *parent,
     current->name = node.name;
     current->isDir = node.children.size();
     if (parent) {
-        current->fullPath = parent->fullPath + current->name;
+        current->fullPath = Utils::FilePath::fromString(parent->fullPath.toString()
+                                                        + current->name);
         parent->childDirectories.push_back(current);
     } else {
         current->fullPath = Utils::FilePath::fromString(current->name);
@@ -279,7 +280,7 @@ static bool needsLink(ProjectExplorer::Tree *node) {
     return !node->isDir && !node->fullPath.toString().startsWith("clang-analyzer-");
 }
 
-class BaseChecksTreeModel : public ProjectExplorer::SelectableFilesModel
+class BaseChecksTreeModel : public ProjectExplorer::SelectableFilesModel // FIXME: This isn't about files.
 {
     Q_OBJECT