SIL: Add support for capturing the dynamic Self metatype

If a closure captures the dynamic 'Self' type, but no value of type 'Self'
(for example, it is possible to have a weak capture of 'self'; if the weak
reference becomes nil, there's no way for the closure to get the dynamic
'Self' type from the value).

In this case, add a hidden argument of type $Self.Type, and pass in the
Self metatype.

Fixes <https://bugs.swift.org/browse/SR-1558> / <rdar://problem/22299905>.

lib/SILGen/SILGenFunction.cpp

@@ -232,6 +232,28 @@ void SILGenFunction::emitCaptures(SILLocation loc,
     canGuarantee = false;
   
   for (auto capture : captureInfo.getCaptures()) {
+    if (capture.isDynamicSelfMetadata()) {
+      // The parameter type is the static Self type, but the value we
+      // want to pass is the dynamic Self type, so upcast it.
+      auto dynamicSelfMetatype = MetatypeType::get(
+          captureInfo.getDynamicSelfType(),
+          MetatypeRepresentation::Thick)
+              ->getCanonicalType();
+      auto staticSelfMetatype = MetatypeType::get(
+          captureInfo.getDynamicSelfType()->getSelfType(),
+          MetatypeRepresentation::Thick)
+              ->getCanonicalType();
+      SILType dynamicSILType = SILType::getPrimitiveObjectType(
+          dynamicSelfMetatype);
+      SILType staticSILType = SILType::getPrimitiveObjectType(
+          staticSelfMetatype);
+
+      SILValue value = B.createMetatype(loc, dynamicSILType);
+      value = B.createUpcast(loc, value, staticSILType);
+      capturedArgs.push_back(ManagedValue::forUnmanaged(value));
+      continue;
+    }
+
     auto *vd = capture.getDecl();
 
     switch (SGM.Types.getDeclCaptureKind(capture)) {