Sema: Workaround for failing swift-corelibs-foundation build on Linux

Previously, hasArgumentType() would silently return false if the
EnumElementDecl had not yet been typechecked, but now,
getArgumentInterfaceType() asserts in this case.

As the FIXME comment suggests, this warrants further investigation,
but for now add the workaround.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -6857,6 +6857,11 @@ static bool checkEnumDeclCircularity(EnumDecl *E, NominalDeclSet &known,
     known.insert(E);
 
   for (auto elt: E->getAllElements()) {
+    // FIXME: Strange that this can happen, it means we're potentially
+    // not diagnosing circularity when we should be.
+    if (!elt->hasInterfaceType())
+      continue;
+
     // skip uninteresting fields.
     if (!elt->getArgumentInterfaceType() || elt->isIndirect())
       continue;