This permits constants of the form `const X = T(A)` to add `X` to the
stringer output for type `T`.
While those constants can be rewritten as `const X T = T(A)`, that
becomes tedious and visually noisy when `T` is a long name. It is quite
easy to address this easy and common case, while not attempting to solve
this with full generality.
Fixes#11581.
Change-Id: Ifb8e43515f05493de190e02577260d94dd851581
Reviewed-on: https://go-review.googlesource.com/c/146577
Run-TryBot: David Symonds <dsymonds@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
