cmd/compile: enable -newescape by default

RELNOTE=yes

The new escape analysis pass is more precise, which for most Go code
should be an improvement. However, it may also break code that
happened to work before (e.g., code that violated the unsafe.Pointer
safety rules).

The old escape analysis pass can be re-enabled with "go build
-gcflags=all=-newescape=false". N.B., it's NOT recommended to mix the
old and new escape analysis passes such as by omitting "all=". While
the old and new escape analysis passes use similar and mostly
compatible metadata, there are cases (e.g., closure handling) where
they semantically differ and could lead to memory corruption errors in
compiled programs.

Fixes #23109.

Change-Id: I0b1b6a6de5e240cb30c87a165f47bb8795491158
Reviewed-on: https://go-review.googlesource.com/c/go/+/170448


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>