ada: Fix recent assertion failure on GPR2

It's the compiler trying to load the nonexistent body of a generic package
when trying to inline a call to an expression function of this package that
has a pre or post-condition (hence the need for -gnata to trigger the ICE).

gcc/ada/

	* contracts.adb (Build_Subprogram_Contract_Wrapper): Do not fiddle
	with the Was_Expression_Function flag. Move a few lines around.