Implement type sugar "T?" for Optional<T>.

- New type representation OptionalTypeRepr.
- New sugared type OptionalType.
- New base type SyntaxSugarType, parent of ArraySliceType and OptionalType.
  These two are the same in a lot of ways.
- The form "T[]?" is forbidden, because it makes "Int[4][2]" oddly
  different from "Int[4]?[2]". The type can be spelled "(T[])?" or
  Optional<T[]>.
- Like Slice, "Optional" is just looked up in the current module. This may
  or may not be the desired behavior in the long run.

<rdar://problem/14666783>

Swift SVN r7100

lib/Serialization/ModuleFormat.h

@@ -239,6 +239,7 @@ namespace decls_block {
     ARRAY_TYPE,
     REFERENCE_STORAGE_TYPE,
     UNBOUND_GENERIC_TYPE,
+    OPTIONAL_TYPE,
 
     TYPE_ALIAS_DECL = 100,
     STRUCT_DECL,
@@ -388,12 +389,16 @@ namespace decls_block {
     BCFixed<1>   // noreturn?
   >;
 
-  using ArraySliceTypeLayout = BCRecordLayout<
-    ARRAY_SLICE_TYPE,
+  template <unsigned Code>
+  using SyntaxSugarTypeLayout = BCRecordLayout<
+    Code,
     TypeIDField, // element type
     TypeIDField  // implementation type
   >;
 
+  using ArraySliceTypeLayout = SyntaxSugarTypeLayout<ARRAY_SLICE_TYPE>;
+  using OptionalTypeLayout = SyntaxSugarTypeLayout<OPTIONAL_TYPE>;
+
   using ArrayTypeLayout = BCRecordLayout<
     ARRAY_TYPE,
     TypeIDField, // element type