[dev.typeparams] cmd/compile: avoid ir.DeepCopy in noder.constDecl

Instead of using ir.DeepCopy to copy the IR from the previous constant
declaration, just call exprList again and then fix up the position
information. This is equivalent in practice, but has cleaner semantics
for tricky corner cases like constant declarations that contain
function literals.

In particular, this refactoring is necessary for the next CL that
cleans up function literal construction, because it adds extra
consistency checks that weren't satisfied by DeepCopy'd OCLOSUREs.

Change-Id: I0372bde5d6613695ee572cc8bf8fb4ff9aef4cb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/327449


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>