[Ada] Add assertions to Uintp

[Ada] Add assertions to Uintp

gcc/ada/

	* uintp.ads, uintp.adb: Add assertions.
	(Ubool, Opt_Ubool): New "boolean" subtypes.
	(UI_Is_In_Int_Range): The parameter should probably be
	Valid_Uint, but we don't change that for now, because it causes
	failures in gigi.
	* sem_util.ads, sem_util.adb (Is_True, Is_False,
	Static_Boolean): Use Opt_Ubool subtype.  Document the fact that
	Is_True (No_Uint) = True.  Implement Is_False in terms of
	Is_True.  We considered changing Static_Boolean to return Uint_1
	in case of error, but that doesn't fit in well with
	Static_Integer.
	(Has_Compatible_Alignment_Internal): Deal with cases where Offs
	is No_Uint. Change one "and" to "and then" to ensure we don't
	pass No_Uint to ">", which would violate the new assertions.
	* exp_util.adb, freeze.adb, sem_ch13.adb: Avoid violating new
	assertions in Uintp.