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/clangfixitoperationsextractor.cpp

@@ -34,6 +34,9 @@
 
 using ClangBackEnd::DiagnosticContainer;
 
+namespace ClangCodeModel {
+namespace Internal {
+
 namespace {
 
 void capitalizeText(QString &text)
@@ -50,7 +53,7 @@ QString tweakedDiagnosticText(const QString &diagnosticText)
     QString tweakedText = diagnosticText;
 
     if (!tweakedText.isEmpty()) {
-        tweakedText = ClangCodeModel::Utils::diagnosticCategoryPrefixRemoved(tweakedText);
+        tweakedText = diagnosticCategoryPrefixRemoved(tweakedText);
         capitalizeText(tweakedText);
     }
 
@@ -71,9 +74,6 @@ bool hasFixItAt(const QVector<ClangBackEnd::FixItContainer> &fixits,
 
 } // anonymous namespace
 
-namespace ClangCodeModel {
-namespace Internal {
-
 ClangFixItOperationsExtractor::ClangFixItOperationsExtractor(
         const QVector<DiagnosticContainer> &diagnosticContainers)
     : diagnosticContainers(diagnosticContainers)