[Ada] Improve performance of Containers.Functional_Base

[Ada] Improve performance of Containers.Functional_Base

This patch modifies the implementation of Functional_Base to damp the
cost of its subprograms at runtime in specific cases. Instead of copying
the entire underlying array to create a new container, containers can
share the same Array_Base attribute. Performance on common use cases of
formal and functional containers is improved with this patch.

2019-08-14  Joffrey Huguet  <huguet@adacore.com>

gcc/ada/

	* libgnat/a-cofuba.ads: Add a Length attribute to type
	Container. Add a type Array_Base which replaces the previous
	Elements attribute of Container.
	(Content_Init): New subprogram. It is used to initialize the
	Base attribute of Container.
	* libgnat/a-cofuba.adb (Resize): New subprogram. It is used to
	resize the underlying array of a container if necessary.
	(=, <=, Find, Get, Intersection, Length, Num_Overlaps, Set,
	Union): Update to match changes in type declarations.
	(Add): Modify body to damp the time and space cost in a specific
	case.
	(Content_Init): New subprogram. It is used to initialize the
	Base attribute of Container.
	(Remove): Modify body to damp the time and space cost in a
	specific case.

From-SVN: r274474