[Patch, fortran] Fix ICE with class array references.

[Mikael Morin]

Hello,

this patch, extracted with some modifications from PR50981 comment #28
[http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50981#c28],
(which has accumulated a lot of things) fixes an ICE noticed in several
PRs with an error like:

internal compiler error: in gfc_conv_descriptor_data_get,
at fortran/trans-array.c:147

internal compiler error: in gfc_conv_descriptor_offset, at
fortran/trans-array.c:210

the problem is a missing "_data" reference (to escape the class 
container) when trying to access a subobject of a class object.

The solution proposed is to replace the call to 
gfc_add_component_ref(expr, "_data") with a call to a new, more general, 
function gfc_fix_class_refs which takes care of adding the "_data" 
component in all references (not only the last one) where it is missing.
Thus, it works
     - in the scalar case: class%array_comp(1), class%scalar_comp
     - with multiple level of components: class%comp%subclass%sub_comp
     - in the array case (but this was working before): class%array_comp(:)
     - in any mix of the above cases.

I have chosen to make it a separate function instead of fixing 
gfc_add_component_ref, so that it can be reused later (maybe...) even if 
we don't want to add a "_data", or "_vptr" or ... component.


W.R.T. the code itself, I think it is rather straightforward.  There is 
an odd thing to prevent a regression in class_41.f03.  See the big 
comment in class_data_ref_missing.


Regression tested on x86_64-unknown-freebsd9.0.  OK for trunk?

Mikael



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