[Runtime] Form metatypes based on mangled names.

Author: DougGregor

stdlib/public/runtime/MetadataLookup.cpp

@@ -281,8 +281,7 @@ class DecodedMetadataBuilder {
   }
 
   BuiltType createMetatypeType(BuiltType instance, bool wasAbstract) const {
-    // FIXME: Implement.
-    return BuiltType();
+    return swift_getMetatypeMetadata(instance);
   }
 
   BuiltType createExistentialMetatypeType(BuiltType instance) const {

test/Runtime/demangleToMetadata.swift

@@ -65,5 +65,10 @@ DemangleToMetadataTests.test("function types") {
   expectEqual(type(of: f2_variadic_inout), _typeByMangledName("yyytd_ytztc")!)
 }
 
+DemangleToMetadataTests.test("metatype types") {
+  expectEqual(type(of: type(of: ())), _typeByMangledName("ytm")!)
+  expectEqual(type(of: type(of: f0)), _typeByMangledName("yycm")!)
+}
+
 runAllTests()