Change the name of cas() in cc to newcase() to avoid a NIX conflict.
cas() is used in cc to create a new Case struct. There is a name
conflict in that cas() is a commonly-used
name for compare and swap. Since cas() is only used internally
in the compiler in 3 places, change the name to avoid a wider
conflict with the NIX runtime. This issue might well come up on
other OSes in the future anyway, as the name is fairly common.

R=rsc
CC=golang-dev
https://golang.org/cl/5294071