swift234

Remove the emitSemantic* methods from TypeLowering and
move the corresponding functionality into SILGen.

I've switched around 'assign' so that it's no longer a
semantic assignment --- that is, so that it expects a properly
lowered value as its operand, not an r-value of the semantic
type.  This actually simplifies quite a lot of code and removes
some ugly special-casing from MemoryPromotion.

Swift SVN r7942