@extern functions preconditions wiped out in presence of unsupported constructs

[mario-bucev]

In the following snippet, the call to ext2 (in test2) does not generate a VC for the precondition of ext2:

import stainless.annotation._

object ExternRequires {

  @extern
  def ext1(x: Int): Unit = {
    require(x >= 10)
    ()
  }

  @extern
  def ext2(x: Int): Unit = {
    require(x >= 10)
    val s = x.toString // Unsupported construct
    ()
  }

  def test1: Unit = ext1(0) // Invalid (as expected)

  def test2: Unit = ext2(0) // Should fail, but no VC gets generated
}

It seems that having constructs that Stainless does not support leads to having the require clauses being dropped (the postconditions are unaffected, though)