Fix bug in assignment to nested packed structure

Fix bug in assignment to nested packed structure

A user at AdaCore found a case where assignment to a nested packed
structure would fail.  The bug is that ada_value_primitive_field
doesn't account for the situation where a field is not packed relative
to its containing structure, but where the structure itself is packed
in its parent.

gdb/ChangeLog
2019-05-01  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_value_primitive_field): Treat more fields as
	bitfields.

gdb/testsuite/ChangeLog
2019-05-01  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/packed_array_assign/aggregates.ads (Nested_Packed): New
	record.
	(NPR): New variable.
	* gdb.ada/packed_array_assign.exp: Add nested packed assignment
	test.