swift234

[func-sig-opts] Expand the definition of a singleton tree to include trees with multiple levels of single field types.

This means that we will not explode things like:

struct A { var b: B }
struct B { var c: C }
struct C { var leaf: LeafType }

This does not add anything since all of the types are layout
compatible.

In terms of effect on dylib size, there is a small reduction (i.e. < 1%) over
all dylibs. The only really significant change is to Foundation which shrinks by
3%.

rdar://21114206

Swift SVN r29780