Decl: allow "setInvalidDecl" for TagDecls

[elsteveogrande]

Summary

For TagDecls, complete or incomplete, allow setInvalidDecl calls.

Other asserts are interfering but we'll deal with them separately.

https://bugs.llvm.org/show_bug.cgi?id=38531

lib/AST/DeclBase.cpp

 #define DECL(DERIVED, BASE) case DERIVED: return #DERIVED;
 #define ABSTRACT_DECL(DECL)
 #include "clang/AST/DeclNodes.inc"
   }
 }
 
 void Decl::setInvalidDecl(bool Invalid) {
   InvalidDecl = Invalid;
-  assert(!isa<TagDecl>(this) || !cast<TagDecl>(this)->isCompleteDefinition());
   if (!Invalid) {
     return;
   }
 
+  assert(!isa<TagDecl>(this) || !cast<TagDecl>(this)->isCompleteDefinition());
+
   if (!isa<ParmVarDecl>(this)) {
     // Defensive maneuver for ill-formed code: we're likely not to make it to
     // a point where we set the access specifier, so default it to "public"
     // to avoid triggering asserts elsewhere in the front end.
     setAccess(AS_public);
   }
 
   // Marking a DecompositionDecl as invalid implies all the child BindingDecl's