[Ada] Fix small discrepancy between -gnatG and -gnatD[G] outputs

[Pierre-Marie de Rodat]

The outputs generated for the same compilation by -gnatG and -gnatD[G]
are not strictly identical: the former lacks a few freeze nodes, which
can be quite confusing.  So this change aligns the former with the
latter.

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

gcc/ada/

	* sprint.ads (Source_Dump): Fix typo in comment.
	* sprint.adb (Source_Dump): Set Dump_Freeze_Null for both -gnatG
	and -gnatD.
	* switch-c.adb (Scan_Front_End_Switches) <'D'>: Remove obsolete
	kludge.
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