Use Function ptr XI calculations for Thin Func ptrs

Author: tbkka

include/swift/Reflection/TypeLowering.h

@@ -180,8 +180,14 @@ class BuiltinTypeInfo : public TypeInfo {
       *extraInhabitantIndex = -1;
       return true;
     }
-    // XXX Has extra inhabitants, so it must be a pointer?
-    return reader.readHeapObjectExtraInhabitantIndex(address, extraInhabitantIndex);
+    // If it has extra inhabitants, it must be a pointer.  (The only non-pointer
+    // data with extra inhabitants is a non-payload enum, which doesn't get here.)
+    // But there are two different conventions, one for function pointers:
+    if (self == TypeConverter::getThinFunctionTypeInfo()) {
+      return reader.readFunctionPointerExtraInhabitantIndex(address, extraInhabitantIndex);
+    } else {
+      return reader.readHeapObjectExtraInhabitantIndex(address, extraInhabitantIndex);
+    }
   }
 
   static bool classof(const TypeInfo *TI) {

stdlib/public/Reflection/TypeLowering.cpp

@@ -746,7 +746,7 @@ TypeConverter::getReferenceTypeInfo(ReferenceKind Kind,
   return TI;
 }
 
-/// Thick functions consist of a function pointer. We do not use
+/// Thin functions consist of a function pointer. We do not use
 /// Builtin.RawPointer here, since the extra inhabitants differ.
 const TypeInfo *
 TypeConverter::getThinFunctionTypeInfo() {