MemAccessUtils: change the interface of getProjectionPath to Optional value

[shajrawi]

Sometions we don't have a projection path for each ref_elem_addr - fixes a compiler crash in Deferred in the Source Compatibility Suite

When checking if two storage locations are distinct from one another, play it safe in such a case.

radar rdar://problem/43403553

[shajrawi]

@swift-ci Please smoke test and merge

[atrick]

How could we not be able to create the ProjectionPath? I'm concerned about the correctness of that case.

Please add a SIL test for that corner case.

[shajrawi]

@atrick I just went with a debugger on the compatibility projects, this is why we don't have a projection path:

  // Do not inspect the body of types with unreferenced types such as bitfields
  // and unions. This is currently only associated with structs.
  if (Start->getType().aggregateHasUnreferenceableStorage() ||
      End->getType().aggregateHasUnreferenceableStorage())

Which means:

  /// Does this struct contain unreferenceable storage, such as C fields that
  /// cannot be represented in Swift?
  bool hasUnreferenceableStorage() const {

As to why this is happening in the case that crashes, I looked at the source code:
we have a Swift class with a C struct as one of its fields, we are doing a begin_access on the C struct.

[atrick]

Good. So we have a SIL unit test now, which we need for any bizarre corner case like this.

This check End->getType().aggregateHasUnreferenceableStorage() definitely doesn't apply to our problem. I have no idea who added it or why, but they should have added a unit test if they didn't want us to come along and remove it.

[atrick]

  if (Start->getType().aggregateHasUnreferenceableStorage() ||
      End->getType().aggregateHasUnreferenceableStorage())

Incidentally, this also looks like a horrible bug to me. If this is something that needs to be checked before constructing a Projection, then it needs to be checked in the Projection constructor so it covers all elements along the projection path except the End, which is the only value we don't need a Projection into.