Clang: Warn of potentially expensive checks

...for the code model.

If a diagnostic configuration with potentially expensive checks is
selected in Options/Preferences > C++ > Code Model, show a warning below
the combo box.

Change-Id: I52c5c2e229fd50c0fd82f70154fb5b727726ba31
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/clangtools/clangtoolsconstants.h

@@ -28,6 +28,7 @@
 namespace ClangTools {
 namespace Constants {
 
+const char SETTINGS_PAGE_ID[] = "Analyzer.ClangTools.Settings";
 const char SETTINGS_ID[] = "ClangTools";
 const char CLANGTIDYCLAZY_RUN_MODE[] = "ClangTidyClazy.RunMode";