cmd/link: make dwarf name slice index self-describing

cmd/link: make dwarf name slice index self-describing

cmd/compile/internal/dwarfgen.createComplexVar does it this way, which
has the nice property of documenting the expected prefix.

This is primarily for newtype, since defgotype checks for the prefix
immediately prior, but I changed both for consistency.

Change-Id: I49fa7c6166bdcbd19aaf91fe3dc20537080afcfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/635177
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>