[Patch,Fortran] Allow for zero-dimension explict-shaped arrays (PR30514)

[Tobias Burnus]

:ADDPATCH fortran:

gfortran failed for:
  integer(1) :: i(1:-1)
  i = 5
and it ends up in an endless loop, as FX found out, for
  integer :: i(2:0), j(1:0)
  integer, parameter :: k(2:0) = 0, l(1:0) = 0
  i = k
  j = l


Note that those arrays are valid:

See 5.1.2.5.1 Explicit-shape array:
"If the upper bound is less than the lower bound, the range is empty, the
extent in that dimension is zero, and the array is of zero size."


Bootstapped and regression tested on x86_64-unknown-linux-gnu.
Ok for the trunk and the usual week+ later for 4.2?
(It also affects 4.1, do we want to fix it there too [after the 4.1.x
release]?)

Tobias
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