SILGen: Pass heap captures by only box.
Now that boxes are typed and projectable, the address no longer has to be passed separately.
For now, this breaks capture promotion, DI, and debug info, which analyze uses of the address param. Will be addressed in upcoming commits:
Swift :: DebugInfo/byref-capture.swift
Swift :: DebugInfo/closure-args.swift
Swift :: DebugInfo/closure-args2.swift
Swift :: DebugInfo/inout.swift
Swift :: DebugInfo/linetable.swift
Swift :: SILPasses/capture_promotion.swift
Swift :: SILPasses/definite_init_diagnostics.swift
