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/languageclient/languageclientmanager.h

@@ -84,6 +84,7 @@ public:
     static Client *clientForDocument(TextEditor::TextDocument *document);
     static Client *clientForFilePath(const Utils::FilePath &filePath);
     static Client *clientForUri(const LanguageServerProtocol::DocumentUri &uri);
+    static const QList<Client *> clientsForProject(const ProjectExplorer::Project *project);
 
     ///
     /// \brief openDocumentWithClient