Clang: Remove project tracking on clangbackend side

...as it is not needed. Just provide the compilation arguments as part
of the Document.

As a side effect, re-initializing the backend after a crash is cheaper
and will not freeze the UI anymore (referenced bug).

Task-number: QTCREATORBUG-21097
Change-Id: I866e25ef1fd5e4d318df16612a7564469e6baa11
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangcompletionassistprocessor.cpp

@@ -577,7 +577,6 @@ void ClangCompletionAssistProcessor::sendFileContent(const QByteArray &customFil
 
     BackendCommunicator &communicator = m_interface->communicator();
     communicator.documentsChanged({{m_interface->fileName(),
-                                    Utf8String(),
                                     Utf8String::fromByteArray(info.unsavedContent),
                                     info.isDocumentModified,
                                     uint(m_interface->textDocument()->revision())}});
@@ -658,12 +657,10 @@ bool ClangCompletionAssistProcessor::sendCompletionRequest(int position,
 
         const Position cursorPosition = extractLineColumn(position);
         const Position functionNameStart = extractLineColumn(functionNameStartPosition);
-        const QString projectPartId = CppTools::CppToolsBridge::projectPartIdForFile(filePath);
         communicator.requestCompletions(this,
                                         filePath,
                                         uint(cursorPosition.line),
                                         uint(cursorPosition.column),
-                                        projectPartId,
                                         functionNameStart.line,
                                         functionNameStart.column);
         setLastCompletionPosition(filePath, position);