stor-layout: Don't create DECL_BIT_FIELD_REPRESENTATIVE for QUAL_UNION_TYPE [PR101062]

stor-layout: Don't create DECL_BIT_FIELD_REPRESENTATIVE for QUAL_UNION_TYPE [PR101062]

> The following patch does create them, but treats all such bitfields as if
> they were in a structure where the particular bitfield is the only field.

While the patch passed bootstrap/regtest on the trunk, when trying to
backport it to 11 branch the bootstrap failed with
atree.ads:3844:34: size for "Node_Record" too small
errors.  Turns out the error is not about size being too small, but actually
about size being non-constant, and comes from:
 /* In a FIELD_DECL of a RECORD_TYPE, this is a pointer to the storage
    representative FIELD_DECL.  */
 #define DECL_BIT_FIELD_REPRESENTATIVE(NODE) \
   (FIELD_DECL_CHECK (NODE)->field_decl.qualifier)

 /* For a FIELD_DECL in a QUAL_UNION_TYPE, records the expression, which
    if nonzero, indicates that the field occupies the type.  */
  #define DECL_QUALIFIER(NODE) (FIELD_DECL_CHECK (NODE)->field_decl.qualifier)
so by setting up DECL_BIT_FIELD_REPRESENTATIVE in QUAL_UNION_TYPE we
actually set or modify DECL_QUALIFIER and then construct size as COND_EXPRs
with those bit field representatives (e.g. with array type) as conditions
which doesn't fold into constant.

The following patch fixes it by not creating DECL_BIT_FIELD_REPRESENTATIVEs
for QUAL_UNION_TYPE as there is nowhere to store them,

Shall we change tree.h to document that DECL_BIT_FIELD_REPRESENTATIVE
is valid also on UNION_TYPE?
I see:
tree-ssa-alias.c-  if (TREE_CODE (type1) == RECORD_TYPE
tree-ssa-alias.c:      && DECL_BIT_FIELD_REPRESENTATIVE (field1))
tree-ssa-alias.c:    field1 = DECL_BIT_FIELD_REPRESENTATIVE (field1);
tree-ssa-alias.c-  if (TREE_CODE (type2) == RECORD_TYPE
tree-ssa-alias.c:      && DECL_BIT_FIELD_REPRESENTATIVE (field2))
tree-ssa-alias.c:    field2 = DECL_BIT_FIELD_REPRESENTATIVE (field2);
Shall we change that to || == UNION_TYPE or do we assume all fields
are overlapping in a UNION_TYPE already?
At other spots (asan, ubsan, expr.c) it is unclear what will happen
if they see a QUAL_UNION_TYPE with a DECL_QUALIFIER (or does the Ada FE
lower that somehow)?

2021-06-18  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101062
	* stor-layout.c (finish_bitfield_layout): Don't add bitfield
	representatives in QUAL_UNION_TYPE.