By using this class we can create ptml::Tags without
XML tags. It is useful in the revng-c part when we
want to generate Plain C. All PTML Tags, from now on,
should be created via this class only.
In addition, port `Yield` library to be using this.
The `ptml` namespace contains utilities for easier PTML manipulation.
These include:
* A few constants (e.g. standard attributes) available in
`revng/PTML/Constants.h`
* The `Tag` class to easily construct html/xml tags with attributes
without having to use format strings.
* The `PTMLIndentedOstream`, a llvm::raw_ostream wrapper that
automatically adds tagged indentation to the output
Alessandro Di Federico