net: add conversion from AddrPort to TCPAddr to complement existing inverse

We already have various member functions of TCPAddr that return an
AddrPort, but we don't have a helper function to go from a AddrPort to a
TCPAddr. UDP has this, but it was left out of TCP. This commit adds the
corresponding function.

Updates #49298.

Change-Id: I85732cf34f47c792fe13a6b4af64fd4b0e85d06a
Reviewed-on: https://go-review.googlesource.com/c/go/+/362596


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>