[serialization] Introduce ModuleID, with codes for special modules.

ModuleID is compatible with IdentifierID, but uses 0 to mean “the builtin module”
and 1 to mean “the current module”. Anything else is a top-level module name,
as an identifier. As an implementation detail, 1 is now never a valid IdentifierID.
(0 remains “the empty string”.)

Using this, simplify the encoding of the owner of a conformance.

Swift SVN r9944

lib/Serialization/ModuleFile.h

@@ -370,10 +370,13 @@ public:
                 Optional<DeclContext *> ForcedContext = {},
                 std::function<void(Decl*)> DidRecord = nullptr);
 
+  /// Returns the appropriate module for the given ID.
+  Module *getModule(serialization::ModuleID MID);
+
   /// Returns the appropriate module for the given name.
   ///
   /// If the name matches the name of the current module, a shadowed module
-  /// is loaded instead. An empty name represents the Builtin module.
+  /// is loaded instead.
   Module *getModule(Identifier name);
 
   /// Reads a substitution record from \c DeclTypeCursor.