Refactor certain functions to make them simpler. and avoid calling
AST.Type.loweredType, which can fail. Instead, access the types of the
function's (SIL) arguments directly.
Correctly handle exploding packs that contain generic or opaque types by using
AST.Type.mapOutOfEnvironment().
@substituted types cause the shouldExplode predicate to be unreliable for AST
types, so restrict it to just SIL.Type. Add test cases for functions that have
@substituted types.
Re-enable PackSpecialization in FunctionPass pipeline.
Add a check to avoid emitting a destructure_tuple of the original function's
return tuple when it is void/().