[flang] Set lower bounds of array section fir.embox to one

Do not use the shift of a fir.embox to set lower bounds if there is
a fir.slice operand. This matches Fortran semantics where lower bounds
of array sections are ones.
Note that in case there is a fir.slice, the array shift may be provided
because it is used to calculate the origin/base address of an array slice.

Add a TODO for substring codegen since I noticed it was not upstreamed
yet and would cause some program to silently compile incorrectly.

Differential Revision: https://reviews.llvm.org/D123123