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)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148262 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/CommandLine.cpp

@@ -292,12 +292,6 @@ static inline bool ProvideOption(Option *Handler, StringRef ArgName,
     break;
   case ValueOptional:
     break;
-
-  default:
-    errs() << ProgramName
-         << ": Bad ValueMask flag! CommandLine usage error:"
-         << Handler->getValueExpectedFlag() << "\n";
-    llvm_unreachable(0);
   }
 
   // If this isn't a multi-arg option, just run the handler.