[serialization] Add support for extensions.

Currently, we are eagerly deserializing extensions. This probably isn't
what we want to do in the long run, but until lookup settles down a bit
it's not worth optimizing.

Swift SVN r6186

lib/Serialization/ModuleFormat.h

@@ -241,6 +241,7 @@ namespace decls_block {
     ONEOF_DECL,
     ONEOF_ELEMENT_DECL,
     SUBSCRIPT_DECL,
+    EXTENSION_DECL,
 
     PAREN_PATTERN = 200,
     TUPLE_PATTERN,
@@ -498,6 +499,15 @@ namespace decls_block {
     // The indices pattern trails the record.
   >;
 
+  using ExtensionLayout = BCRecordLayout<
+    EXTENSION_DECL,
+    TypeIDField, // base type
+    DeclIDField, // context decl
+    BCFixed<1>,  // implicit flag
+    BCArray<TypeIDField> // inherited types
+    // Trailed by conformance info (if any), then the decl context record.
+  >;
+
   
   using ParenPatternLayout = BCRecordLayout<
     PAREN_PATTERN