[flang] Downgrade error message to a portability warning (#98368)

f18 current emits an error when an assignment is made to an array
section with a vector subscript, and the array is finalized with a
non-elemental final subroutine. Some other compilers emit this error
because (I think) they want variables to only be finalized in place, not
by a subroutine call involving copy-in & copy-out of the finalized
elements.

Since many other Fortran compilers can handle this case, and there's
nothing in the standards to preclude it, let's downgrade this error
message to a portability warning.

This patch got complicated because the API for the WhyNotDefinable()
utility routine was such that it would return a message only in error
cases, and there was no provision for returning non-fatal messages. It
now returns either nothing, a fatal message, or a non-fatal warning
message, and all of its call sites have been modified to cope.

0 files changed, 0 insertions, 0 deletions