crypto/cipher: replace NewGCMWithNonceAndTagSize with NewGCMWithTagSize

This function was added during the Go 1.11 dev cycle and isn't part of
the API compatibility promise yet.

In the previous implementation, NewGCMWithNonceAndTagSize was being used
as a helper function for NewGCM, NewGCMWithTagSize, and NewGCMWithNonceSize.
With the removal of Nonce size from the name and parameters, we needed to
add an unexported helper function newGCMWithNonceAndTagSize.

Fixes #24977

Change-Id: Ie70f2a192d0556c4f890deb62e68cff6bbbccd33
Reviewed-on: https://go-review.googlesource.com/116435


Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>