Clang: Use completion through backend process

This makes us independent of libclang crashes for completion.
Re-parsing for highlighting still happens in the Qt Creator process.

Run in verbose mode:
    qtc.clangcodemodel.ipc=true

Run tests:
    -test "ClangCodeModel"

Task-number: QTCREATORBUG-14108
Task-number: QTCREATORBUG-12819
Change-Id: Id3e95bd2afdb6508bbd1d35fddc69534a909b905
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>

src/plugins/cpptools/cppcompletionassistprocessor.h

@@ -48,7 +48,7 @@ public:
 protected:
     void addSnippets();
 
-    int m_startPosition;
+    int m_positionForProposal;
     QList<TextEditor::AssistProposalItem *> m_completions;
     QStringList m_preprocessorCompletions;
     TextEditor::IAssistProposal *m_hintProposal;