XFAIL a compiler_crashers regression

This now fails with the infamous 'Assertion failed: ((conforms ||
replacement->is<ErrorType>() || firstArchetype->getIsRecursive() ||
isOpenedAnyObject(replacement) || replacement->is<GenericTypeParamType>())
&& "Constraint system missed a conformance?")' error.

The actual problem is something with unbound generics and inference
of generic parameters. Here's a minimal test case:

protocol A {}

struct B<f> {}

func a<T:A>() {
  class AA {
    var t:B=B()
  }
}

I'll fix it soon.

Author: slavapestov

validation-test/compiler_crashers_fixed/28269-swift-expr-walk.swift

@@ -7,4 +7,5 @@
 
 // RUN: not %target-swift-frontend %s -parse
 // REQUIRES: asserts
+// XFAIL: *
 protocol A.struct B<f func a<T:A{class A{var t:B=