Add -Wswitch-enum-redundant-default.

This warning acts as the complement to the main -Wswitch-enum warning (which
warns whenever a switch over enum without a default doesn't cover all values of
 the enum) & has been an an-doc coding convention in LLVM and Clang in my
experience. The purpose is to ensure there's never a "dead" default in a
switch-over-enum because this would hide future -Wswitch-enum errors.

The name warning has a separate flag name so it can be disabled but it's grouped
under -Wswitch-enum & is on-by-default because of this.

The existing violations of this rule in test cases have had the warning disabled
& I've added a specific test for the new behavior (many negative cases already
exist in the same test file - and none regressed - so I didn't add more).

Reviewed by Ted Kremenek ( http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20120116/051690.html )

llvm-svn: 148640