[CSSimplify] Relax transitive conformance check

`ModuleDecl::lookupConformance` defaults `allowMissing` to `false`
which is too restrictive for the solver because we need to accept
missing but synthesizable cases and filter them later via solution
ranking.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -8650,7 +8650,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyTransitivelyConformsTo(
 
   // First, let's check whether the type itself conforms,
   // if it does - we are done.
-  if (M->lookupConformance(resolvedTy, protocol))
+  if (M->lookupConformance(resolvedTy, protocol, /*allowMissing=*/true))
     return SolutionKind::Solved;
 
   // If the type doesn't conform, let's check whether