Create writer for new JSON format.
The new JSON format allows practically all attributes to be defined per
target in a universal library however the internal representation only
allows one for the time being. For now the write will always write those
attributes as default available for all targets (install name,
compatability & current version, swift abi, flags e.g. flatnamepace &
app exenstion safety)
rdar://102076911
Reviewed By: ributzka
Differential Revision: https://reviews.llvm.org/D144339
d6f9b97bae