[Ada] Do not perform useless work in Check_No_Parts_Violations

[Pierre-Marie de Rodat]

There is no need to walk the hierarchy rooted at a type that does not
come from source only to drop the result on the floor.  No functional
changes.

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

gcc/ada/

	* freeze.adb (Check_No_Parts_Violations): Return earlier if the
	type is elementary or does not come from source.
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