[dev.link] cmd/link: implement live method tracking in deadcode2

[dev.link] cmd/link: implement live method tracking in deadcode2

This essentially replicates the logic of live method tracking and
type symbol decoding, rewritten to operate on indices instead of
Symbols.

TODO: the special handling of reflect.Type.Method has not been
implemented.

TODO: the symbol name is used too much. It ought to be a better
way to do it.

Change-Id: I860ee7a506c00833902e4870d15aea698a705dd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/199078
Reviewed-by: Than McIntosh <thanm@google.com>