Small fix to PLACEHOLDER_EXPR handling in loc_list_from_tree_1

Small fix to PLACEHOLDER_EXPR handling in loc_list_from_tree_1

This handles the discriminated record types of Ada: the PLACEHOLDER_EXPR is
the "template" expression for the discriminant in the type definition. Now
for some components, typically arrays whose upper bound is the discriminant,
the compiler creates a local subtype for the component, so the code needs to
be able to deal with this nested type.

gcc/ChangeLog:
	* dwarf2out.c (loc_list_from_tree_1) <PLACEHOLDER_EXPR>: Deal with
	a nested context type