[Ada] Do not perform null exclusion static checks on generated entities

[Pierre-Marie de Rodat]

Performing null exclusion checks on generated entities can result in
bogus error messages.

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

gcc/ada/

	* sem_ch3.adb (Analyze_Object_Declaration): Add
	Comes_From_Source call.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.diff
Type: text/x-diff
Size: 372 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc-patches/attachments/20200716/00d57fa7/attachment-0001.bin>