Set @param-taint for Parser::internalParse()

Set @param-taint for Parser::internalParse()

This is not strictly accurate, because Parser::internalParse() actually
returns half-parsed HTML, which is not safe for output. But it is safe for
output from a parser tag.

Maybe phan-taint-check plugin needs to learn about half-parsed HTML as an
extra taint type, and make that an acceptable thing for parser tags to return,
but not other things.

But this fixes the failures for the Listings extension, so I think it's
worthwhile in the meantime.

Change-Id: Idf87f5c3dcf81dd210de73a4ff15e3b1aabd9f89