Previously, EmitPolymorphicParameters dealt directly with an Explosion
from which it pulled values. In one place, there was a conditional
check for async which handled some cases. There was however another
place where the polymorphic parameter was pulled directly from the
explosion. That missed case resulted in attempting to pull a
polymorphic parameter directly from an Explosion which contains only a
%swift.context* per the async calling convention.
Here, those parameters are now pulled from an EntryPointArgumentEmission
subclasses of which are able to provide the relevant definition of what
pulling a parameter means.
rdar://problem/70144083
