assert(0) -> LLVM_UNREACHABLE.

Make llvm_unreachable take an optional string, thus moving the cerr<< out of
line.
LLVM_UNREACHABLE is now a simple wrapper that makes the message go away for
NDEBUG builds.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75379 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/CommandLine.cpp

@@ -693,7 +693,7 @@ void cl::ParseCommandLineOptions(int argc, char **argv,
           ValNo++;
           break;
         default:
-          assert(0 && "Internal error, unexpected NumOccurrences flag in "
+          LLVM_UNREACHABLE("Internal error, unexpected NumOccurrences flag in "
                  "positional argument processing!");
         }
       }