QDoc creates truncated temporary header files

Trying to `make docs` resulted in lots of errors like this:

./Qt-5.15.11_build/qtbase/src/gui/painting/qbrush.cpp:661: (qdoc) warning: clang found diagnostics parsing \fn void QBrush::swap(QBrush &other)
    error: use of undeclared identifier 'QBrush'
    error: unknown type name 'QBrush'

The build log also showed lots of "QIODevice::write: device not open" entries.

It turned out that QDoc created lots of truncated header files, by closing a QFile before the QTextStream operating on it had flushed all its buffered content (see https://codereview.qt-project.org/gitweb?p=qt/qttools.git;a=blob;f=src/qdoc/clangcodeparser.cpp;h=2caa39edeaa153b7e3f245b82d92530f960766aa;hb=HEAD#l1428)

The attached patch fixes the issue.

I encountered the issues with Qt 5.15.11 building on macOS Monterey with XCode 13.4.1 (SDK 12.3), but I think this affects all Qt versions and all platforms.