go/constant: share the empty string
This saves 11,000 allocations when loading the standard library
in golang.org/x/tools/go/ssa form.
Change-Id: I8aa32b0641c1a3dde29e6ee76c760006035dd56d
Reviewed-on: https://go-review.googlesource.com/c/go/+/423934
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>