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/ModuleFile.cpp

@@ -1892,6 +1892,20 @@ Type ModuleFile::getType(TypeID TID) {
     break;
   }
 
+  case decls_block::OPTIONAL_TYPE: {
+    TypeID baseID, implID;
+    decls_block::OptionalTypeLayout::readRecord(scratch, baseID, implID);
+
+    auto optionalTy = OptionalType::get(getType(baseID), ctx);
+    typeOrOffset = optionalTy;
+
+    // Optional types are uniqued by the ASTContext, so they may already have
+    // type information. If so, ignore the information in the module.
+    if (!optionalTy->hasImplementationType())
+      optionalTy->setImplementationType(getType(implID));
+    break;
+  }
+
   case decls_block::ARRAY_TYPE: {
     TypeID baseID;
     uint64_t size;