Move -Wswitch-enum to -Wswitch

This matches GCC's documented (& actual) behavior. What Clang had implemented
as -Wswitch-enum was actually GCC's -Wswitch behavior. -Wswitch is on by
default (part of -Wall) and warns if a switch-over-enum, without a default
case, covers all enum values.

-Wswitch-enum, on the other hand, does not have the default clause and should
fire even in the presence of a default. This warning is off by default.

With this change the -Wswitch-enum flag is off-by-default in Clang but has no
functionality at the moment. I'll add that in a future commit.

llvm-svn: 148648