[flang] Upstream the lowering of the while loop

[flang] Upstream the lowering of the while loop

Upstream the code for handling while loops from the fir-dev branch at
https://github.com/flang-compiler/f18-llvm-project/tree/fir-dev/

Also add tests.

The while loop is lowered to a header block that checks the loop
condition and branches either to the exit block or to the body of the
loop. The body of the loop will unconditionally branch back to the
header.

Differential Revision: https://reviews.llvm.org/D126636

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: V Donaldson <vdonaldson@nvidia.com>