Clang: Move unexported classes to Internal

Change-Id: I3ea197b734f146d4b11431aaf927aed9be7d8756
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangpreprocessorassistproposalitem.cpp

@@ -30,6 +30,7 @@
 #include <cplusplus/Token.h>
 
 namespace ClangCodeModel {
+namespace Internal {
 
 bool ClangPreprocessorAssistProposalItem::prematurelyApplies(const QChar &typedCharacter) const
 {
@@ -156,4 +157,5 @@ bool ClangPreprocessorAssistProposalItem::isInclude() const
         || m_completionOperator == CPlusPlus::T_ANGLE_STRING_LITERAL;
 }
 
+} // namespace Internal
 } // namespace ClangCodeModel