First, we need to set base.Pos in varDecl() and typeDecl(), so it will
be correct if we need to report type size errors while converting types.
Changed error calls in types/sizes.go to use Errorf, not ErrorfAt, since
we want to use base.Pos (which will set from t.Pos(), if that is
available).

Second, we need to add an extra call CalcSize(t1.Elem()) in the
TCHANARGS case of CalcSize(). We can use CalcSize() rather than
CheckSize(), since we know the top-level recursive type will have been
calculated by the time we process the fake TCHANARGS type. In -G=0 mode,
the size of the channel element has often been calculated because of
some other processing (but not in the case of #49767). But in -G=3 mode,
we just calculate sizes during the single noder2 pass, so we are more
likely to have not gotten to calculating the size of the element yet,
depending on the order of processing of the deferredTypeStack.

Fixes the tests fixedbugs/issue{42058a,42058b}.go that were
disabled for -G=3 mode.

Had to add exceptions in stdlib_test.go for go/types and types2, because
the types2 typechecker does not know about type size limits.

Fixes #49814
Fixes #49771
Updates #49767

Change-Id: I77d058e8ceff68a58c4c386a8cf46799c54b04c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/367955
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>