[dev.typeparams] cmd/compile: fix escape printout bugs for -G=3

Call SetPos() in g.expr() so it is available for any new nodes.

Print out the actual type for a composite literal in exprFmt() if
available, else use Ntype if available. Seems generally useful, since
the type name is always more useful than just 'composite literal'.

Fixes a bunch of cases that are excluded in run.go for -G=3.

Change-Id: I40b9bba88027ea4f36d419e3989e7f14891bea04
Reviewed-on: https://go-review.googlesource.com/c/go/+/334609


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>