[release-branch.go1.18] cmd/compile: replace Type.OrigSym with Type.OrigType

[release-branch.go1.18] cmd/compile: replace Type.OrigSym with Type.OrigType

First law of cmd/compile frontend development: thou shalt not rely on
types.Sym.

This CL replaces Type.OrigSym with Type.OrigType, which semantically
matches what all of the uses within the frontend actually care about,
and avoids using types.Sym, which invariably leads to mistakes because
symbol scoping in the frontend doesn't work how anyone intuitively
expects it to.

Fixes #51855.

Change-Id: I4affe6ee0718103ce5006ab68aa7e1bb0cac6881
Reviewed-on: https://go-review.googlesource.com/c/go/+/394274
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit adae6ec542c3287ffe643093a0f61c9871f4e238)
Reviewed-on: https://go-review.googlesource.com/c/go/+/394296