[Ada] Remove obsolete uses of Unchecked_Conversion from Ada 83

[Ada] Remove obsolete uses of Unchecked_Conversion from Ada 83

Similar to a recent removal of obsolete uses of Unchecked_Deallocation.

In Ada 83 the Unchecked_Conversion was a top-level unit; since Ada 95 it
is an obsolete renaming of Ada.Unchecked_Conversion. GNAT doesn't warn
yet about uses of these obsolete renamings, but it still seems better to
avoid them.

gcc/ada/

	* atree.adb, gen_il-gen.adb, osint.adb, set_targ.adb,
	sinput.adb, table.adb, treepr.adb, types.ads: Replace uses of
	Unchecked_Conversion in the compiler itself.
	* libgnarl/a-reatim.adb, libgnarl/s-osinte__gnu.ads,
	libgnarl/s-osinte__kfreebsd-gnu.ads, libgnat/a-coboho.adb,
	libgnat/a-stuten.ads, libgnat/s-putima.adb: Likewise in the
	runtime.
	* doc/gnat_ugn/gnat_and_program_execution.rst: Likewise in
	documentation.
	* gnat_ugn.texi: Regenerate.