[Ada] Overhaul code implementing conversions involving fixed-point types

[Ada] Overhaul code implementing conversions involving fixed-point types

This ovehauls the code implementing conversions involving fixed-point
types in the front-end because it leaks the Do_Range_Check flag in
several places to the back-end, which is a violation of the documented
interface between front-end and back-end.

This also does a bit of housekeeping work throughout it in the process.

There should be essentially no functional changes.

2019-07-22  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* checks.adb (Apply_Type_Conversion_Checks): Do not set
	Do_Range_Check flag on conversions from fixed-point types
	either.
	* exp_attr.adb: Add use and with clause for Expander.
	(Expand_N_Attribute_Reference) <Fixed_Value, Integer_Value>: Set
	the Conversion_OK flag and do not generate overflow/range checks
	manually.
	* exp_ch4.adb (Expand_N_Qualified_Expression): Remove
	superfluous clearing of Do_Range_Check flag.
	(Discrete_Range_Check): New procedure to generate a range check
	for discrete types.
	(Real_Range_Check): Remove redundant local variable and adjust.
	Remove useless shortcut.  Clear Do_Range_Check flag on all
	paths.
	(Expand_N_Type_Conversion): Remove redundant test on
	Conversion_OK.  Call Discrete_Range_Check to generate range
	checks on discrete types.  Remove obsolete code for
	float-to-integer conversions.  Add code to generate range checks
	for conversions involving fixed-point types.

From-SVN: r273692