[demangler] Use _LIBCPP_UNREACHABLE()

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@309649 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/cxa_demangle.cpp b/src/cxa_demangle.cpp
index 2904204..e13358b 100644
--- a/src/cxa_demangle.cpp
+++ b/src/cxa_demangle.cpp
@@ -896,7 +896,7 @@
     case SpecialSubKind::iostream:
       return StringView("basic_iostream");
     }
-    return StringView();
+    _LIBCPP_UNREACHABLE();
   }
 
   void printLeft(OutputStream &S) const override {
@@ -945,7 +945,7 @@
     case SpecialSubKind::iostream:
       return StringView("iostream");
     }
-    return StringView();
+    _LIBCPP_UNREACHABLE();
   }
 
   void printLeft(OutputStream &S) const override {