d: Fix ICE in gimplify_expr, at gimplify.c (PR98277)

d: Fix ICE in gimplify_expr, at gimplify.c (PR98277)

The DMD front-end shouldn't, but can sometimes leak manifest constants
in the AST passed to the code generator.  To prevent this being an
issue, the setting of DECL_INITIAL has been moved to the point where the
CONST_DECL is used, rather than in the declaration handler.

gcc/d/ChangeLog:

	PR d/98277
	* decl.cc (DeclVisitor::visit (VarDeclaration *)): Move setting of
	DECL_INITIAL for manifest constants to ...
	(get_symbol_decl): ... here.

gcc/testsuite/ChangeLog:

	PR d/98277
	* gdc.dg/pr98277.d: New test.