[flang] Fix one corner case in reshape intrinsic

As Fortran 2018 16.9.163, the reshape is the only intrinsic which
requires the shape argument to be rank-one integer array and the SIZE
of it to be one constant expression. The current expression lowering
converts the shape expression with slice in intrinsic into one box value
with the box element type of unknown extent. However, the genReshape
requires the box element type to be constant size. So, convert the box
value into one with box element type of sequence of 1 x constant. This
corner case is found in cam4 in SPEC 2017
https://github.com/llvm/llvm-project/issues/56140.

Reviewed By: Jean Perier

Differential Revision: https://reviews.llvm.org/D128597