[Ada] Ensure constructor is a C++ constructor

[Pierre-Marie de Rodat]

Freeze_Subprogram has special handling of C++ constructor interfacing.
This is currently implicit but assuming we introduce other conventions
(e.g. Java) the current code won't hold.

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

2019-10-10  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* freeze.adb (Freeze_Subprogram): Ensure constructor is a C++
	constructor.
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