swift234

Give Builtin.Word an abstract size.

Instead of hardcoding Builtin.Word to be an alias for Builtin.Int64, make it its own type of abstract pointer width.

- Change BuiltinIntegerType's width representation to accommodate abstract widths.

- In the AST and in SIL, store values of the types as the greatest supported size for the abstract width (64 bits for a pointer).

- Add some type safety to the ([sz]ext|trunc)(OrBitCast)? builtins that they're used appropriately given the upper and lower bounds of the abstract sizes they're working with.

- Now that Builtin.Word is a distinct type, give it its own mangling.

- In IRGen, lower pointer-sized BuiltinIntegerType appropriately for the target, and truncate lowered SIL values if necessary.

Fixes <rdar://problem/15367913>.

Swift SVN r10467