Ada: Fix spurious warning for renaming of component of VFA record

This is a regression present on the mainline and all active branches: the
compiler gives a spurious "is not referenced" warning for the renaming of
a component of a Volatile_Full_Access record.

gcc/ada/
	PR ada/107536
	* exp_ch2.adb (Expand_Renaming): Mark the entity as referenced.

gcc/testsuite/
	* gnat.dg/renaming18.adb: New test.