[flang] Add AllocatableInit functions for use in allocate lowering

`AllocatableInitIntrinsic`, `AllocatableInitCharacter` and
`AllocatableInitDerived` are meant to be used to initialize a
descriptor when it is instantiated and not to be used multiple
times in a scope.
Add `AllocatableInitDerivedForAllocate`, `AllocatableInitCharacterForAllocate`
and `AllocatableInitDerivedForAllocate` to be used for the allocation
in allocate statement.
These new functions are meant to be used on an initialized descriptor
and will return directly if the descriptor is allocated so the
error handling is done by the call to `AllocatableAllocate`.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D146290