[Ada] Address potentially uninitialized variables and dead code

2020-06-02  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* bcheck.adb, binde.adb, bindo-diagnostics.adb, checks.adb,
	exp_aggr.adb, exp_ch3.adb, exp_ch4.adb, exp_ch7.adb,
	exp_ch9.adb, gnatname.adb, sem_case.adb, sem_ch13.adb,
	sem_ch5.adb, sem_prag.adb, sem_util.adb, uintp.adb, urealp.adb,
	xoscons.adb, xr_tabls.adb, xref_lib.adb: Initialize objects more
	explicitly and add corresponding assertions. Remove dead code.
	Also add a few Annotate pragmas to help static analysis.
	* libgnat/a-caldel.adb, libgnat/a-calend.adb,
	libgnat/a-ngcoty.adb, libgnat/a-ngelfu.adb,
	libgnat/a-ngrear.adb, libgnat/a-strfix.adb,
	libgnat/g-calend.adb, libgnat/g-catiio.adb,
	libgnat/g-comlin.adb, libgnat/g-debpoo.adb,
	libgnat/g-dirope.adb, libgnat/g-hesorg.adb,
	libgnat/g-pehage.adb, libgnat/g-socket.adb, libgnat/i-cobol.adb,
	libgnat/s-dwalin.adb, libgnat/s-dwalin.ads,
	libgnat/s-fatgen.adb, libgnat/s-gearop.adb,
	libgnat/s-genbig.adb, libgnat/s-imgrea.adb,
	libgnat/s-os_lib.adb, libgnat/s-rannum.adb,
	libgnat/s-regpat.adb, libgnat/s-trasym__dwarf.adb,
	libgnat/s-valrea.adb: Ditto.