CppTools: Add dedicated settings and settings page for clangd

We plan to add more clangd settings, and it makes sense to have a
dedicated place for them both in the code and the UI.

Change-Id: Ideb92935b7a5a6a98e07980f4011736fb82042d1
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdclient.cpp

@@ -382,7 +382,7 @@ public:
 
 static BaseClientInterface *clientInterface(const Utils::FilePath &jsonDbDir)
 {
-    Utils::CommandLine cmd{CppTools::codeModelSettings()->clangdFilePath(),
+    Utils::CommandLine cmd{CppTools::ClangdSettings::clangdFilePath(),
                            {"--background-index", "--limit-results=0"}};
     if (!jsonDbDir.isEmpty())
         cmd.addArg("--compile-commands-dir=" + jsonDbDir.toString());