swift234

IRGen: Preliminary resilient struct support

Add a new ResilientStructTypeInfo. This is a singleton since
all resilient structs have opaque payloads and are accessed
through value witness tables.

With this in place, flesh out IRGenModule::isResilient() and
use the new singleton to convert resilient structs.

Note that the old isResilient() was hard-coded to report that
all Clang-imported classes are "resilient". Now that this has
been unified with NominalTypeDecl::hasFixedLayout(), we will
report Clang-imported classes are "resilient" at the SIL level.
This should not introduce any semantic differences at this
point.

Unlike SIL, where currently resilient types are always resilient
even when used from the same module, IRGen is able to perform
direct manipulation of resilient structs from the current
module, since IRGen's type lowering has a resilience scope
plumbed through.

Note that we do not yet support laying out structs and classes
containing resilient fields -- this will come in a future patch.