swift234

Strawman @fixed_layout attribute and -{enable,disable}-resilience flags

A fixed layout type is one about which the compiler is allowed to
make certain assumptions across resilience domains. The assumptions
will be documented elsewhere, but for the purposes of this patch
series, they will include:

- the size of the type
- offsets of stored properties
- whether accessed properties are stored or computed

When -enable-resilience is passed to the frontend, all types become
resilient unless annotated with the @fixed_layout attribute.

So far, the @fixed_layout attribute only comes into play in SIL type
lowering of structs and enums, which now become address-only unless
they are @fixed_layout. For now, @fixed_layout is also allowed on
classes, but has no effect. In the future, support for less resilient
type lowering within a single resilience domain will be added, with
appropriate loads and stores in function prologs and epilogs.

Resilience is not enabled by default, which gives all types fixed
layout and matches the behavior of the compiler today. Since
we do not want the -enable-resilience flag to change the behavior
of existing compiled modules, only the currently-compiling module,
Sema adds the @fixed_layout flag to all declarations when the flag
is off. To reduce the size of .swiftmodule files, this could become
a flag on the module itself in the future.

The reasoning behind this is that the usual case is building
applications and private frameworks, where there is no need to make
anything resilient.

For the standard library, we can start out with resilience disabled,
while perfoming an audit adding @fixed_layout annotations in the
right places. Once the implementation is robust enough we can then
build the standard library with resilience enabled.