[Ada] Eliminate redundant overflow checks for conversions from fixed-point

[Ada] Eliminate redundant overflow checks for conversions from fixed-point

This eliminates redundant overflow checks that are generated for
conversions from fixed-point to integer types when range checks are also
enabled (which is the default), as the former checks are subsumed into
the latter checks.

No functional changes.

2019-07-23  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* checks.adb (Activate_Overflow_Check): Remove redundant
	argument.
	* exp_ch4.adb (Discrete_Range_Check): Reset the overflow flag.
	(Expand_N_Type_Conversion): Do not reset it here.

From-SVN: r273724