swift234

IRGen: Add FixedTypeInfo methods for extra inhabitants/spare bits.

Dynamic forms of these methods will ultimately move to the fully general TypeInfo class, but for now I'm focusing on getting fixed-layout unions working. Set up methods on FixedTypeInfo for getting the fixed count of extra inhabitants and the spare bit mask for a type. Have the default implementation of extra inhabitants count use the spare bits. For now, only actually set spare bits for types with IR-level single scalar non-power-of-two integer representation (e.g., i1, or i21).

Swift SVN r7289