[flang] Support -f[no-]realloc-lhs. (#120165)
-frealloc-lhs is the default.
If -fno-realloc-lhs is specified, then an allocatable on the left
side of an intrinsic assignment is not implicitly (re)allocated
to conform with the right hand side. Fortran runtime will issue
an error if there is a mismatch in shape/type/allocation-status.
