Removing unused default switch cases in switches over enums that already...

Removing unused default switch cases in switches over enums that already account for all enumeration values explicitly.

(This time I believe I've checked all the -Wreturn-type warnings from GCC & added the couple of llvm_unreachables necessary to silence them. If I've missed any, I'll happily fix them as soon as I know about them)

llvm-svn: 148262