Implement enough of the 'auto' keyword so we can claim to support N2546.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74307 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/PCHWriter.cpp b/lib/Frontend/PCHWriter.cpp
index 808df70..e93219e 100644
--- a/lib/Frontend/PCHWriter.cpp
+++ b/lib/Frontend/PCHWriter.cpp
@@ -1909,6 +1909,9 @@
     case BuiltinType::NullPtr:    ID = pch::PREDEF_TYPE_NULLPTR_ID;    break;
     case BuiltinType::Overload:   ID = pch::PREDEF_TYPE_OVERLOAD_ID;   break;
     case BuiltinType::Dependent:  ID = pch::PREDEF_TYPE_DEPENDENT_ID;  break;
+    case BuiltinType::UndeducedAuto:
+      assert(0 && "Should not see undeduced auto here");
+      break;
     }
 
     Record.push_back((ID << 3) | T.getCVRQualifiers());