[dev.typeparams] cmd/compile: cleanup unified IR file format a little

[dev.typeparams] cmd/compile: cleanup unified IR file format a little

This CL makes two changes:

1. It moves object symbols and code tags into a new "relocName"
relocation, which should eventually allow getting rid of objStub.

2. It moves the type parameter data into the relocObjDict relocation,
so everything related to writing out dictionaries is contained there.

Change-Id: If0f7ff7d9384e8664957c3180bf6f20e97bcff6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/336051
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>