[Ada] Fix incorrect placement of freeze node with predicate

[Pierre-Marie de Rodat]

This prevents the freezing mechanism from putting a node inside the
subprogram body generated for a predicate function, which can for
example happen for a function referenced in the predicate.

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

gcc/ada/

	* freeze.adb (In_Expanded_Body): Return true for the body of a
	generated predicate function.
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