[flang][runtime] Add API to help with the difficult array constructor cases

This runtime API can be used to lower any flavor of array constructors,
but is mainly intended to be used with:

- array constructors for which the extent or length parameters cannot
 be computed without lowering some ac-value or ac-implied-do-control
 that cannot be pre-evaluated.

- array constructors of a derived type with allocatable component where
 copy is not trivial or PDTS.

Example of use cases:
 - `[((i+j,i=1, ifoo()), j=1,n)]` where ifoo() is not pure.
 - `[return_allocatable_array(), return_allocatable_array()]`

Differential Revision: https://reviews.llvm.org/D144411