Move -Wcovered-switch-default out of -Wswitch (and -Wall), and make it an opt-in warning.

This is a great warning, but it was observed that a ton of real world code violates
it all the time for (semi-)legitimate reasons.  This warnings is fairly pedantic, which is good,
but not for everyone.  For example, there is a fair amount of idiomatic code out there
that does "default: abort()", and similar idioms.

Addresses <rdar://problem/10814651>.

llvm-svn: 150055