[golang/go] x/tools/go/ssa: generics support (#48525)

DRANK

This is a tracking issue for supporting generics within the x/tools/go/ssa package. This depends heavily on proposals for "go/ast" https://github.com/golang/go/issues/47781 and "go/types" https://github.com/golang/go/issues/47916. A reasonable goal is to have ssa available at roughly the same time as the release of generics so tool authors can take advantage of it. Earlier so tool authors that build on top of ssa can support generics at the same time as the release would be even better.