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/cppcompletionassistprovider.h

@@ -43,7 +43,7 @@ QT_END_NAMESPACE
 namespace CPlusPlus { struct LanguageFeatures; }
 
 namespace TextEditor {
-class BaseTextEditor;
+class TextEditorWidget;
 class AssistInterface;
 }
 
@@ -61,7 +61,7 @@ public:
 
     virtual TextEditor::AssistInterface *createAssistInterface(
             const QString &filePath,
-            QTextDocument *document,
+            const TextEditor::TextEditorWidget *textEditorWidget,
             const CPlusPlus::LanguageFeatures &languageFeatures,
             int position,
             TextEditor::AssistReason reason) const = 0;