decl.c (gnat_to_gnu_field): Emit a specialized diagnostic for component size mismatch wrt volatile requirements.

	* gcc-interface/decl.c (gnat_to_gnu_field): Emit a specialized
	diagnostic for component size mismatch wrt volatile requirements.
	Add a gcc_unreachable() at the end of the checks for size.  Split
	the check on volatile for positions into one check on atomic and
	a subsequent one on volatile.

From-SVN: r194946