[dev.typeparams] cmd/compile/internal/types: review of typestring.go

The changes between (equivalent, and reviewed) go/types/typestring.go
and typestring.go can be seen by comparing patchset 1 and 3. The actual
change is just removing the "// UNREVIEWED" marker plus an adjustment
to writeTParamList (we now always write type constraints).

Change-Id: Ieb109c17756addc954e1ca0da606fa5b335ff30d
Reviewed-on: https://go-review.googlesource.com/c/go/+/293472


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>