This change prevents the analysis phase of the front-end from setting
the Do_Range_Check flag in the very peculiar case of the source of a
conversion whose result is passed by reference to a "valued procedure",
because the expansion phase would not be able to generate the check.

This pattern appears in the ancient DEC Starlet package and it doesn't
seem to be useful at this point to change the expander to deal with it,
so instead the analysis phase is adjusted.  Morever the compiler already
issues a warning in this case so this is probably good enough.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-08-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_res.adb: Add with & use clause for Sem_Mech and
	alphabetize.
	(Resolve_Actuals): Do not apply a scalar range check for the
	source of a conversion whose result is passed by reference to a
	valued procedure.