[Ada] Fix bogus alignment warning on address clause

[Eric Botcazou]

This is a regression present on the mainline and 10 branch: the compiler gives 
a bogus alignment warning on an address clause and a discriminated record type 
with variable size.

Tested on x86_64-suse-linux, applied on the mainline and 10 branch.


2020-09-28  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/decl.c (maybe_saturate_size): Add ALIGN parameter
	and round down the result to ALIGN.
	(gnat_to_gnu_entity): Adjust calls to maybe_saturate_size.


2020-09-28  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/addr16.adb: New test.
	* gnat.dg/addr16_pkg.ads: New helper.

-- 
Eric Botcazou
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