[Ada] Return a valid value when handling Constraint_Error

[Pierre-Marie de Rodat]

We hit the Constraint_Error because Start_Column is outside bounds.  So
it should not be returned because that will again raise a
Constraint_Error. Instead return its type'Last which is then properly
handled.

Also reformat the Blanks_Loop to be a simple loop.  It seems, the nested
if's were written for performance consideration.  This does not seem do
be relevant anymore.

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

gcc/ada/

	* scng.adb (Set_Start_Column): Return Column_Number'Last when
	handling Constraint_Error. Plus reformat a strangely 7 level if
	loop.
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