swift234

SIL.rst: Update per discussion with Chris and John
- Tighten up terminology, for instance, define what a "box" actually is
- Describe how operands can have zero, one, or many values
- Describe that only %operands are valid operands and loading constants, ints, and other literals requires distinct insns
- Make releasing a reftype and destroying a value distinct
- Describe how stack allocation of boxes and reftypes work
- Name address-only type operations `destroy_addr` and `copy_addr` to avoid confusion with rvalue operations
- Describe the full set of aggregate manipulation functions
- Clean up instruction notation to avoid ambiguities
- Throw in notes about things we need to design at some point

Swift SVN r3211