Fix MSVC "not all control paths return a value" warning. NFC.
diff --git a/clang/lib/AST/NestedNameSpecifier.cpp b/clang/lib/AST/NestedNameSpecifier.cpp
index 206e462..d3195e6 100644
--- a/clang/lib/AST/NestedNameSpecifier.cpp
+++ b/clang/lib/AST/NestedNameSpecifier.cpp
@@ -289,6 +289,7 @@
     // These are not representable as types.
     return nullptr;
   }
+  llvm_unreachable("Unhandled SpecifierKind enum");
 }
 
 /// Print this nested name specifier to the given output