[Ada] Fix bogus error on conversion from Float to 128-bit unsigned

[Pierre-Marie de Rodat]

The compiler confuses itself because it generates a range check in Float
whose upper bound is 2 ** 128, but this number is larger than the largest
number representable in Float so it triggers an error when converted to
Float as part of the check.

The fix is to saturate the upper bound of the check to that of Float.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* checks.adb (Apply_Float_Conversion_Check): Saturate the bounds
	of the check to those of the base type of the expression.
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