ClangTools: Prefer .clang-tidy file by default

... and move this setting outside the diagnostic config.

Fixes: QTCREATORBUG-28852
Change-Id: Ie3b19ba7bec2bc96451f3216fa06a6941cad4c94
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangtools/clangtoolsutils.cpp

@@ -343,5 +343,13 @@ QString clazyDocUrl(const QString &check)
     return QString::fromLatin1(urlTemplate).arg(versionString, check);
 }
 
+bool toolEnabled(CppEditor::ClangToolType type, const ClangDiagnosticConfig &config,
+                 const RunSettings &runSettings)
+{
+    if (type == ClangToolType::Tidy && runSettings.preferConfigFile())
+        return true;
+    return config.isEnabled(type);
+}
+
 } // namespace Internal
 } // namespace ClangTools