[Ada] Fix missing check for no-op conversion to fixed-point type

[Ada] Fix missing check for no-op conversion to fixed-point type

This plugs a small loophole in the compiler for the case of a
multiplication or a division in a fixed-point type wrapped in a no-op
conversion, e.g. to the same fixed-point type.

The front-end fails to generate a range check for the operation.  This
used to be caught by the back-end, which would generate the range check,
but this is no longer the case because we now make sure to reset the
Do_Range_Check flag in all cases before invoking the back-end.

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

gcc/ada/

	* exp_ch4.adb (Expand_N_Type_Conversion): Beef up comment.
	(Fixup_Universal_Fixed_Operation): Set the base type instead of
	the type of the enclosing type conversion on the operation.

gcc/testsuite/

	* gnat.dg/fixedpnt6.adb: New testcase.

From-SVN: r273695