swift234

[docs] Move some sections in SIL.rst to match SILNodes.def and add some stub docs/full docs for more instructions.

Specifically:

I Moved ref_to_unowned and unowned_to_ref to conversion inst section so
we match SILNodes.def.

I added full docs for:

  1. unchecked_trivial_bit_cast
  2. unchecked_ref_bit_cast

and stub docs for:

  1. objc_protocol.
  2. autorelease_value.
  3. init_block_storage_header
  4. project_block_header

Swift SVN r21671