[Sema] Avoid null-node crash for invalid closure parameter wrappers

[Flamki]

Summary

This fixes an assertion crash when a closure parameter uses an invalid property wrapper.

For code like:

@propertyWrapper
struct A {}

let c = { (@A x) in
  return x
}

the compiler was asserting in ConstraintSystem::setType because closure wrapper-synthesized vars could be missing, but resolveClosure unconditionally attempted to assign types to them.

What changed

• In ConstraintSystem::resolveClosure (lib/Sema/CSSimplify.cpp):
  • Guard use of closure wrapper-synthesized vars (backingVar, wrappedValueVar).
  • Only run wrapper-specific type assignment and wrapper constraints when those vars are present.
• In applySolutionToClosurePropertyWrappers (lib/Sema/CSSyntacticElement.cpp):
  • Guard interface type assignment for wrapper vars the same way.
• Added regression coverage in test/Sema/property_wrapper_parameter_invalid.swift for Usage of property wrapper in global closure parameter causes assertion failure #87479.

Why this approach

Invalid wrapper declarations are already diagnosed by declaration checking. In this path we only need to avoid crashing while closure constraints are being resolved/applied.

Before / After

Before:

• Assertion failure during type checking (Cannot set type information on null node).

After:

• No null-node assignment in closure wrapper handling when synthesized vars are missing.
• Existing wrapper diagnostic is preserved, and regression coverage is added.

Issue

Fixes #87479

[Flamki]

@xedin Friendly ping on this when you have a moment.

This PR fixes #87479 (null-node crash with invalid closure parameter wrappers), and required checks are still in “Waiting for status to be reported”.

If it looks good, could someone trigger Swift CI for it?