Index: lib/Basic/SourceManager.cpp
===================================================================
--- lib/Basic/SourceManager.cpp
+++ lib/Basic/SourceManager.cpp
@@ -2034,6 +2034,9 @@
 }
 
 /// \brief Determines the order of 2 source locations in the translation unit.
+///        It also works when two locations are from different translation
+///        units. In that case it will return *some* order, that is
+///        deterministic for that invocation of the compiler.
 ///
 /// \returns true if LHS source location comes before RHS, false otherwise.
 bool SourceManager::isBeforeInTranslationUnit(SourceLocation LHS,
@@ -2130,7 +2133,8 @@
       return LIsScratch;
     return LOffs.second < ROffs.second;
   }
-  llvm_unreachable("Unsortable locations found");
+  // Source locations from different translation units.
+  return LOffs.first < ROffs.first;
 }
 
 void SourceManager::PrintStats() const {