[Ada] Spurious error on parameterless acccess_to_subprogram

[Pierre-Marie de Rodat]

Compiler rejects an indirect call through an Access_To_Subprogram
value that denotes a parameterless subprogram, when the corresponding
access type has a pre- or postcondition.

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

gcc/ada/

	* exp_ch3.adb (Build_Access_Subprogram_Wrapper_Body): Create a
	proper signature when the access type denotes a parameterless
	subprogram.
	* exp_ch6.adb (Expand_Call): Handle properly a  parameterless
	indirect call when the corresponding access type has contracts.
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