swift234

IRGen: Layout of root classes containing resiliently-sized fields

Now that all the machinery is in place, the ClassMetadataBuilder
can (more accurately) query the ClassLayout instead of trying to
re-derive whether the field offset vector is dependent, etc.

Apart from performing dynamic layout for resiliently-sized fields
in concrete classes, this also lets us *skip* dynamic layout
if we have a generic class without any dependent fields.

I haven't tested subclassing with resilient field layout yet, but
getting that working is the next step and should not be too much
work.

Also, swift_initClassMetadata_UniversalStrategy() only stores
the computed field offsets in the field offset globals when the
Objective-C runtime is available, because it gets the offset
pointers from the Objective-C class rodata. On Linux, we will
need to emit code to copy from the field offset vector into
field offset globals in IRGen. This is pretty easy, but I'll do
it in a follow-up patch so for now the new execution test is
XFAIL'd on Linux.