[libc++] Implement P0627R6 (Function to mark unreachable code)

Reviewed By: ldionne, Quuxplusone, #libc

Spies: arichardson, mstorsjo, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D119152

NOKEYCHECK=True
GitOrigin-RevId: 2a8f9a5e95dec3adcdee603358dc03def6d2245e
diff --git a/src/strstream.cpp b/src/strstream.cpp
index e62c077..fe7e2d4 100644
--- a/src/strstream.cpp
+++ b/src/strstream.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__utility/unreachable.h>
 #include "strstream"
 #include "algorithm"
 #include "climits"
@@ -268,7 +269,7 @@
             newoff = seekhigh - eback();
             break;
         default:
-            _LIBCPP_UNREACHABLE();
+            __libcpp_unreachable();
         }
         newoff += __off;
         if (0 <= newoff && newoff <= seekhigh - eback())