swift234

IRGen: Use metadata accessors for types with resilient layout

If a struct has fixed layout but contains fields which are opaque,
or if the struct itself is opaque, use a metadata accessor function
instead of loading the metadata directly.

Let's say that A and B are two structs defined in the same module,
and B has a fixed size. This patch adds support for these two cases:

1) Fixed-layout struct A contains resilient struct B
2) Resilient struct A contains resilient struct B

In case 1),

a) Inside X: A is fixed-size and has constant metadata
 i) Direct metadata access can be performed on A and B
 ii) Direct field access can be performed on A and B
d) Outside X: B has an opaque layout
 i) Metadata accessor must be called for A and B
 ii) Fields of A do not have constant offsets, instead the offsets
     must be loaded from type metadata

Case 2) is the same as above except ii) does not apply, since fields
of resilient structs are manipulated via accessors.

Eventually, we will use metadata accessor functions for all public
struct and enum types.