Simplify destruction of Modules in LLVContextImpl.

This avoids copying the container by simply deleting until empty.

While I'd rather move to a stricter ownership semantic (unique_ptr),
SmallPtrSet can't cope with unique_ptr and the ownership semantics here
are a bit incestuous (Module sort of owns itself, but sort of doesn't
(if the LLVMContext is destroyed before the Module, then it deregisters
itself from the context... )).

Ideally Modules would be given to the context, or possibly an
emplace-like function to construct them there. Modules then shouldn't be
destroyed by LLVM API clients, but by interacting with the owner
(LLVMContext) directly (but even then, passing a Module* to LLVMContext
doesn't provide an easy way to destroy the Module, since the set would
be over unique_ptrs and you'd need a heterogenous lookup function which
SmallPtrSet doesn't have either).