cmd/compile/internal/types2: unexport Context.TypeHash

Context.TypeHash is not being used outside of the type checker, so
unexport it.

The TypeHash method is meant to hash instances, not arbitrary types, and
will soon be modified to differentiate origin types by pointer identity
(even if they are *Signature types).

Change-Id: Ia8d4a7c6350ce7f278b70630585efb0009fef63a
Reviewed-on: https://go-review.googlesource.com/c/go/+/363516


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>