[Sema] Remove an isInvalid check

This check was added in 38e305c,
and was used to detect whether there may be
something invalid about the members that were
necessary for conformance derivation. However this
is no longer the case, a nominal should only ever
be marked invalid these days if it is either in an
unsupported DeclContext, or it's a redeclaration.
The synthesis logic ought to be resilient against
invalid members.

Additionally, this check wasn't even necessarily
looking at the nominal, it could have been
looking at an ExtensionDecl. In that case, we could
have still attempted to synthesize with invalid
members. As such, it seems unlikely this check is
providing any real benefit, let's remove it.

Author: hamishknight

lib/Sema/TypeCheckProtocol.cpp

@@ -4511,11 +4511,6 @@ deriveProtocolRequirement(const NormalProtocolConformance *Conformance,
   if (!derivableKind)
     return nullptr;
 
-  auto *DC = Conformance->getDeclContext();
-  const auto Decl = DC->getInnermostDeclarationDeclContext();
-  if (Decl->isInvalid())
-    return nullptr;
-
   DerivedConformance derived(Conformance, TypeDecl, protocol);
 
   switch (*derivableKind) {