Using a type containing a sync type directly
in a function call (whether as a receiver,
a param, or a return value) is an easy way
to accidentally copy a lock or other sync primitive.
Check for it.
The test as implemented does not provide 100%
coverage; see the discussion near the bottom of
testdata/copylock.go for shortcomings.
Fixesgolang/go#6729.
R=adg, r, dsymonds
CC=golang-dev
https://golang.org/cl/23420043
