ClangCodeModel: Replace nested ClangCodeModel::Utils namespace

... by the customary ::Internal.

It adds only noise on the user side and conflicts regularly with
the top-level ::Utils namespace.

Remove a (now) duplicated definition of setLastSentDocumentRevision().

Plus minor namespace related fixes.

There are still minor conflicts between Utils::Text and
ClangCodeModel::Text

Change-Id: I2e8df6b3c6c3599192774032822ee7e778355bba
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangcodemodel/clangdiagnosticmanager.cpp

@@ -324,7 +324,7 @@ static void addTask(const ClangBackEnd::DiagnosticContainer &diagnostic, bool is
     }
 
     TaskHub::addTask(Task(taskType,
-                          Utils::diagnosticCategoryPrefixRemoved(diagnostic.text.toString()),
+                          diagnosticCategoryPrefixRemoved(diagnostic.text.toString()),
                           FilePath::fromString(diagnostic.location.filePath.toString()),
                           diagnostic.location.line,
                           Constants::TASK_CATEGORY_DIAGNOSTICS,