When compiling concurrently, we walk all functions before compiling
any of them. Walking functions can cause variables to switch from
being non-addrtaken to addrtaken, e.g. to prepare for a runtime call.
Typechecking propagates addrtaken-ness of closure variables to
their outer variables, so that capturevars can decide whether to
pass the variable's value or a pointer to it.

When all functions are compiled immediately, as long as the containing
function is compiled prior to the closure, this propagation has no effect.
When compilation is deferred, though, in rare cases, this results in 
a change in the addrtaken-ness of a variable in the outer function,
which in turn changes the compiler's output.
(This is rare because in a great many cases, a temporary has been
introduced, insulating the outer variable from modification.)
But concurrent compilation must generate identical results.

To fix this, track whether capturevars has run.
If it has, there is no need to update outer variables
when closure variables change.
Capturevars always runs before any functions are walked or compiled.

The remainder of the changes in this CL are to support the test.
In particular, -d=compilelater forces the compiler to walk all
functions before compiling any of them, despite being non-concurrent.
This is useful because -live is fundamentally incompatible with
concurrent compilation, but we want -c=1 to have no behavior changes.

Fixes #20250

Change-Id: I89bcb54268a41e8588af1ac8cc37fbef856a90c2
Reviewed-on: https://go-review.googlesource.com/42853
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>