[Runtime] Lookup @objc protocols in mangled name -> type metadata function.

Swift-defined @objc protocols are registered with the Objective-C runtime
under the Swift 3 mangling scheme; look in the Objective-C runting using
objc_getProtocol() with the appropriate name.

Also, correctly compute the "class bound" bit when forming a protocol
composition metatype. The information isn't in the mangled name when it
can be recovered from the protocols themselves, so look at the protocols.

test/Runtime/demangleToMetadataObjC.swift

@@ -0,0 +1,30 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+import StdlibUnittest
+import Foundation
+
+let DemangleToMetadataTests = TestSuite("DemangleToMetadataObjC")
+
+@objc class C : NSObject { }
+@objc enum E: Int { case a }
+@objc protocol P1 { }
+
+DemangleToMetadataTests.test("@objc classes") {
+  expectEqual(type(of: C()), _typeByMangledName("4main1CC")!)
+}
+
+DemangleToMetadataTests.test("@objc enums") {
+  expectEqual(type(of: E.a), _typeByMangledName("4main1EO")!)
+}
+
+func f1_composition_objc_protocol(_: P1) { }
+
+DemangleToMetadataTests.test("@objc protocols") {
+  expectEqual(type(of: f1_composition_objc_protocol),
+              _typeByMangledName("yy4main2P1_pc")!)
+}
+
+runAllTests()
+