[dev.typeparams] cmd/compile/internal/types2: import regexp/syntax instead of cmd...

[dev.typeparams] cmd/compile/internal/types2: import regexp/syntax instead of cmd/compile/internal/syntax

This is a straight port of https://golang.org/cl/330431.

For #43232

Change-Id: I5954bdff22a524eaa08754947da9b428b27f7d95
Reviewed-on: https://go-review.googlesource.com/c/go/+/336351
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>