swift234

[SIL] Adopt llvm::TrailingObjects where it makes sense for SIL instructions.

SIL already has a pretty good infrastructure for tail-allocated operands, although
it's not enforced in any way. SwitchEnumInstBase would also benefit from this if
we were willing to make it a template, or if we had a way to override the 'final'
requirement on llvm::TrailingObjects.