Index: lib/Basic/SourceManager.cpp
===================================================================
--- lib/Basic/SourceManager.cpp
+++ lib/Basic/SourceManager.cpp
@@ -2035,7 +2035,10 @@
       return LIsScratch;
     return LOffs.second < ROffs.second;
   }
-  llvm_unreachable("Unsortable locations found");
+
+  // If SourceLocations originate from different TUs, but cannot be handled
+  // by one of the previous cases, just compare the FileIDs directly.
+  return LOffs.first < ROffs.first;
 }
 
 std::pair<bool, bool> SourceManager::isInTheSameTranslationUnit(