ada: Fix compile-time failure due to duplicated attribute subprograms.

ada: Fix compile-time failure due to duplicated attribute subprograms.

For a given type, and for certain attributes (the 4 streaming attributes
and, for Ada2022, the Put_Image attribute), the compiler needs to keep track
of whether a subprogram has already been generated for the given
type/attribute pair. In some cases this was being done incorrectly;
the compiler ended up generating duplicate subprograms (with the same
name), resulting in compilation failures. This could occur if the prefix
of an attribute reference denoted a subtype (more precisely, a non-first
subtype). This includes the case of a subtype declaration that is implicitly
introduced by the compiler to capture the binding between a formal type
in a generic and the corresponding actual type in an instantiation.

gcc/ada/ChangeLog:

	* exp_attr.adb (Expand_N_Attribute_Reference): When accessing the
	maps declared in package Cached_Attribute_Ops, the key value
	passed to Get or to Set should never be the entity node for a
	subtype. Use the entity of the corresponding type declaration
	instead.