ClangCodeModel: Add experimental clangd support

If the user has enabled clangd (default is off), we start up one instance
per project when it is opened/changed (including build config switches),
and trigger background indexing.
So far, the index is used to provide results for locators and "Find
Usages".
Per-document functionality such as semantic highlighting and completion
is still provided by libclang.

Change-Id: I12532fca1b9c6278baab560e7238cba6189cde9f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangmodelmanagersupport.h

@@ -47,6 +47,8 @@ class FollowSymbolInterface;
 class RefactoringEngineInterface;
 } // namespace CppTools
 
+namespace LanguageClient { class Client; }
+
 namespace ClangCodeModel {
 namespace Internal {
 
@@ -77,6 +79,8 @@ public:
 
     ClangProjectSettings &projectSettings(ProjectExplorer::Project *project) const;
 
+    LanguageClient::Client *clientForProject(const ProjectExplorer::Project *project);
+
     static ClangModelManagerSupport *instance();
 
 private:
@@ -118,6 +122,11 @@ private:
     void connectToTextDocumentContentsChangedForUnsavedFile(TextEditor::TextDocument *textDocument);
     void connectToWidgetsMarkContextMenuRequested(QWidget *editorWidget);
 
+    void updateLanguageClient(ProjectExplorer::Project *project,
+                              const CppTools::ProjectInfo &projectInfo);
+    LanguageClient::Client *createClient(ProjectExplorer::Project *project,
+                                         const Utils::FilePath &jsonDbDir);
+
 private:
     UiHeaderOnDiskManager m_uiHeaderOnDiskManager;
     BackendCommunicator m_communicator;