[Ada] Fixes for various wrong and missing error messages on ACATS B85100[567]

[Ada] Fixes for various wrong and missing error messages on ACATS B85100[567]

GNAT was missing errors as well as issuing messages on legal lines in
new ACATS tests for illegal renamings of discriminant-dependent
components. Cases that are fixed include object names involving
qualified expressions, dereference cases, and generic formal access and
formal derived types. Better implements the "known to be constrained"
rules in the Ada RM.

Tested by new ACATS tests B85100[567] that are soon to be released.

2018-12-03  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* sem_aux.adb (Object_Type_Has_Constrained_Partial_View): Return
	True for an untagged discriminated formal derived type when
	referenced within a generic body (augments existing test for
	formal private types).
	* sem_util.adb (Is_Dependent_Component_Of_Mutable_Type): If the
	prefix of the name is a qualified expression, retrieve the
	operand of that. Add a test of whether the (possible)
	dereference prefix is a variable, and also test whether that
	prefix might just be of an access type (occurs in some implicit
	dereference cases) rather than being an explicit dereference.
	Retrieve the Original_Node of the object name's main prefix and
	handle the possibility of that being a qualified expression.
	Remove special-case code for explicit dereferences that don't
	come from source. Add test for the renaming not being within a
	generic body for proper determination of whether a formal access
	type is known to be constrained (it is within a generic spec,
	but not in the body).  Fix an existing incorrect test for
	renaming of a discriminant-dependent component of a untagged
	generic formal type being within a generic body, adding test of
	taggedness and calling In_Generic_Body (now properly checks for
	cases where the renaming is in a nongeneric body nested within a
	generic).  Return False in cases where the selector is not a
	component (or discriminant), which can occur for
	prefixed-notation calls.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@266759 138bc75d-0d04-0410-961f-82ee72b054a4