[CHERI] Fix an assertion failure when in containsCapabilities when a member is an incomplete record.

This can only arise when an error has already occurred earlier in parsing.

Author: resistor

clang/lib/AST/ASTContext.cpp

@@ -13558,9 +13558,14 @@ bool ASTContext::containsCapabilities(const RecordDecl *RD) const {
     const QualType Ty = i->getType();
     if (Ty->isCHERICapabilityType(*this))
       return true;
-    if (const RecordType *RT = Ty->getAs<RecordType>())
+    if (const RecordType *RT = Ty->getAs<RecordType>()) {
+      if (RT->isIncompleteType())
+        // This can only occur when an error has occurred earlier, so it
+        // isn't too important what we return.
+        return false;
       if (containsCapabilities(RT->getDecl()))
         return true;
+    }
     if (Ty->isArrayType() && containsCapabilities(Ty))
       return true;
   }