In some common cases, a reference to Some_Type'Some_Streaming_Attribute
causes the needed subprogram to be generated "on demand". If there are
multiple such references (e.g., two calls to Some_Type'Write) then we
want to avoid generating multiple essentially-identical subprograms.
This change implies that a generated streaming subprogram may now have
multiple call sites, so we can no longer use the source position information
from the (one and only) call site. If an exception is raised during a
streaming operation, this can make a difference in the reported raise location.

gcc/ada/

	* exp_attr.adb
	(Cached_Streaming_Ops): A new package, providing maps to save
	previously-generated Read/Write/Input/Output procedures.
	(Expand_N_Attribute_Reference): When a new subprogram is generated
	for a Read/Write/Input/Output attribute reference, record that
	type/subp pair in the appropriate Cached_Streaming_Ops map.
	(Find_Stream_Subprogram): Check the appropriate
	Cached_Streaming_Ops map to see if an appropriate subprogram has
	already been generated. If so, then return it. The appropriateness
	test includes a call to a new nested subprogram,
	In_Available_Context.
	* exp_strm.ads, exp_strm.adb: Do not pass in a Loc parameter (or a
	source-location-bearing Nod parameter) to the 16 procedures
	provided for building streaming-related subprograms. Use the
	source location of the type instead.
	* exp_dist.adb, exp_ch3.adb: Adapt to Exp_Strm spec changes. For
	these calls the source location of the type was already being
	used.