go/types, types2: add unifyMode to unifier, pass it through
Pass a mode parameter through all unifier calls but make no use of it.
When unifying type elements (components of composite types), use emode,
which currently is set to mode.
Preparatory step to fix #60460. Factoring out this mechanical change
will make the actual fix smaller and easier to review and understand.
Because this change doesn't affect the behavior of the unifier, it is
safe.
For #60460.
Change-Id: I5b67499d93025be2128c14cc00bcc3b8cc9f44b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/498955
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>