d: Fix internal compiler error: in visit, at d/decl.cc:838 [PR119799]

d: Fix internal compiler error: in visit, at d/decl.cc:838 [PR119799]

This was caused by a check in the D front-end disallowing static
VAR_DECLs with a size `0'.

While empty structs in D are give the size `1', the same symbol coming
from ImportC modules do infact have no size, so allow C variables to
pass the check as well as array objects.

	PR d/119799

gcc/d/ChangeLog:

	* decl.cc (DeclVisitor::visit (VarDeclaration *)): Check front-end
	type size before building the VAR_DECL.  Allow C symbols to have a
	size of `0'.

gcc/testsuite/ChangeLog:

	* gdc.dg/import-c/pr119799.d: New test.
	* gdc.dg/import-c/pr119799c.c: New test.