[Ada] Use "aspect" instead of "pragma" in warnings

[Ada] Use "aspect" instead of "pragma" in warnings

For warnings related to the Unreferenced, Unmodified, and Unused
aspects, refer to the "aspect", not the "pragma". Note that the aspect
can be set by an aspect_specification or a pragma, so the term "aspect"
is correct even in the pragma case.  However, messages in sem_prag.adb
use Fix_Error, so they refer to "aspect" or "pragma". It's not clear
that we really ought to refer to "pragma", now that those are obsolete,
but de call Fix_Error for uniformity. We do not attempt to make
Fix_Error available outside sem_prag.adb -- too much trouble for
something that perhaps ought to be removed.

gcc/ada/

	* lib-xref.adb, sem_util.adb: Change wording of warning messages
	to use "aspect".
	* sem_prag.adb: Use Fix_Error to change wording of warnings.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Minor: fix
	cut&paste error.
	* gnat_rm.texi: Regenerate.