[dev.regabi] cmd/compile: move setUnderlying to package types

[dev.regabi] cmd/compile: move setUnderlying to package types

Now that setUnderlying is decoupled from Nodes, it can be moved into
package types, where it really belongs.

[git-generate]
cd src/cmd/compile/internal/gc
rf '
mv setUnderlying SetUnderlying
mv SetUnderlying typex.go
mv typex.go cmd/compile/internal/types
'

cd ../types
rf '
mv typex.go type.go
'

Change-Id: I76e2d4d8a6df599f24a731c4d8e5774ec83a119c
Reviewed-on: https://go-review.googlesource.com/c/go/+/274433
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>