ClangCodeModel: Clear existing libclang diagnostics

... when clangd takes over a document.
This fixes the problem that temporary diagnostics persisted after opening
a session.

Change-Id: Ic781f83747cabb4d15c2c1f8181e36f4343e0394
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangmodelmanagersupport.cpp

@@ -316,6 +316,7 @@ void ClangModelManagerSupport::updateLanguageClient(ProjectExplorer::Project *pr
                 if (!project->isKnownFile(entry->fileName()))
                     continue;
                 client->openDocument(textDocument);
+                ClangEditorDocumentProcessor::clearTextMarks(textDocument->filePath());
                 hasDocuments = true;
             }