swift234

[func-sig-opts] Restrict the set of functions that we will optimize.

The sets not being considered are:

1. Any functions that will always be inlined. We ignore the inlined case since we can just
optimize the signature of the caller at each of the given function call sites.

2. Functions that have already been specialized once.

3. External functions. This is because for function signature
optimization to work, we need to be able to modify the caller and the
callee. Thus if the caller or callee is external since we can not modify
the external definition, we can not perform any specialization.

4. Generic functions. I am leaving this for a later iteration.

5. Non-Fragile functions. I am leaving this for a later iteration.

6. ObjCMethods and Witness Methods. We can never perform this
optimization on ObjCMethods and I am going to enable witness methods in
a later iteration.

7. Functions without call sites. Since we are only specializing fragile
functions, if a function without call sites is used in a different module, that
module will be able to deserialize it. Once I handle non-fragile functions, this
will change.

Swift SVN r22564