Remove redundant break statements, and replace asserts with llvm_unreachable

Per John's review comments for r140068.

llvm-svn: 140142
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 52a05db..6ad8584 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6148,8 +6148,7 @@
     Error = 4;
     break;
   case SC_OpenCLWorkGroupLocal:
-    assert(0 && "Unexpected storage class");
-    break;
+    llvm_unreachable("Unexpected storage class");
   }
   // FIXME: constexpr isn't allowed here.
   //if (DS.isConstexprSpecified())