GPCJ is a Java port of Alan Murta's GPC, which is in C. The C GPC code is still available though not from the original site. Calling that C code from Swift should not be difficult.
https://github.com/rickbrew/GeneralPolygonClipper
But:
I only wish to draw the convex hull of the typhoon path in maps
Well you don't want a polygon clipper, you want a convex hull algorithm!
I would consider using the version in Boost.Geometry (C++), with a suitable wrapper.
Beware, it is easy to write a bad convex hull implementation due to the numerical stability issues.