IRGen: Fix enum lowering with -enable-resilience-bypass

[slavapestov]

The layout of an enum type will only use spare bits if the
payload types have a fixed size in all resilience domains
where they are visible. In practice, this means that:

• If the enum is internal or resilient, we can use spare bits
  if the payload types are fixed size from inside the current
  module.

• If the enum is public and not resilient, we can use spare bits
  if the payload types are fixed size from all resilience
  domains.

The bug was that the 'fixed size in all resilience domains'
check was returning true for resilient types when the
-enable-resilience-bypass flag was on. This is wrong, because
this meant that building a module with and without
-enable-resilience-bypass could produce different lowerings
for enum types.

Fixes rdar://problem/40034143.

[slavapestov]

@swift-ci Please test

[slavapestov]

@jrose-apple Please review for 4.2

[jrose-apple]

I've convinced myself the logic in the description makes sense, but I don't (yet) see how that corresponds to the code change. What makes internal and resilient enums use Maximal but public non-resilient enums use Minimal here? How does this affect other callers using Minimal?

[jrose-apple]

Slava came to explain this in person. The short form is that this is like saying "pretend LLDB is inside the current module, but don't pretend the entire world is non-resilient". This is closer to how IRGen normally behaves, and does of course fix this issue. So, LGTM.

[slavapestov]

That’s a good explanation, thanks.