[dev.typeparams] cmd/compile: point StructKeyExpr at the types.Field

When constructing struct literals, importers need a way to specify
precisely which field to initialize without worrying about visibility
or those fields being blank. (A blank field doesn't actually need to
be initialized, but the expression needs to be evaluated still, and
with the right order-of-operations.)

This CL changes StructKeyExpr's Field field to point directly to the
corresponding types.Field, rather than merely holding a copy of its
Sym and Offset. This is akin to past changes to add
SelectorExpr.Selection.

Change-Id: I95b72b1788f73206fcebc22b456cf6b1186db6a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/325031


Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>