swift234

Add a global_init attribute to SILFunction.

The implied semantics are:
- side-effects can occur any time before the first invocation.
- all calls to the same global_init function have the same side-effects.
- any operation that may observe the initializer's side-effects must be
  preceded by a call to the initializer.

This is currently true if the function is an addressor that was lazily
generated from a global variable access. Note that the initialization
function itself does not need this attribute. It is private and only
called within the addressor.

Swift SVN r16683