[flang] add evaluate::IsAllocatableDesignator helper

Previously, some semantic checks that are checking if an entity is an
allocatable were relying on the expression being a designator whose
last symbol has the allocatable attribute.

This is wrong since this was considering substrings and array sections of
allocatables as being allocatable. This is wrong (see NOTE 2 in
Fortran 2018 section 9.5.3.1).

Add evaluate::IsAllocatableDesignator to correctly test this.
Also add some semantic tests for ALLOCATED to test the newly added helper.
Note that ifort and nag are rejecting coindexed-named-object in
ALLOCATED (`allocated(coarray_scalar_alloc[2])`).
I think it is wrong given allocated argument is intent(in) as per
16.2.1 point 3.
So 15.5.2.6 point 4 regarding allocatable dummy is not violated (If the actual
argument is a coindexed object, the dummy argument shall have the INTENT (IN)
attribute.) and I think this is valid. gfortran accepts it.

The need for this helper was exposed in https://reviews.llvm.org/D122779.

Differential Revision: https://reviews.llvm.org/D122899



Co-authored-by: Peixin-Qiao <qiaopeixin@huawei.com>