[Coroutines] Do not add allocas for retcon coroutines

Same as for async-style lowering, if there are no resume points in a
function, the coroutine frame pointer will be replaced by an undef,
making all accesses to the frame undefinde behavior.

Fix this by not adding allocas to the coroutine frame if there are no
resume points.

Differential Revision: https://reviews.llvm.org/D137866