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/cppeditor/cppeditor.cpp

@@ -617,7 +617,7 @@ AssistInterface *CppEditorWidget::createAssistInterface(AssistKind kind, AssistR
             features.objCEnabled = cppEditorDocument()->isObjCEnabled();
             return cap->createAssistInterface(
                             textDocument()->filePath().toString(),
-                            document(),
+                            this,
                             features,
                             position(),
                             reason);