cmd/gc: fix corruption in export of &T{} literals.

Composite literals using the &T{} form were incorrectly
exported, leading to weird errors at import time.

Fixes issue 4879.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7395054
cmd/gc: accept cases with same value but different types in switch.

Fixes issue 4781.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7365056