d: Set up input_location before lowering Dsymbol AST

d: Set up input_location before lowering Dsymbol AST

Like a similarly named function in the visitor class for statements,
this ensures that the current input_location is set to the correct
source file location of the decl.

It is likely that there are a number of cases where declarations have
ended up with no location without this.

gcc/d/ChangeLog:

	* decl.cc (DeclVisitor::build_dsymbol): New function.
	(DeclVisitor::visit (TupleDeclaration *)): Use build_dsymbol to
	traverse AST instead of accept.
	(DeclVisitor::visit (AttribDeclaration *)): Likewise.
	(DeclVisitor::visit (Nspace *)): Likewise.
	(DeclVisitor::visit (TemplateDeclaration *)): Likewise.
	(DeclVisitor::visit (TemplateInstance *)): Likewise.
	(DeclVisitor::visit (TemplateMixin *)): Likewise.
	(DeclVisitor::visit (StructDeclaration *)): Likewise.
	(DeclVisitor::visit (ClassDeclaration *)): Likewise.
	(DeclVisitor::visit (InterfaceDeclaration *)): Likewise.
	(DeclVisitor::visit (VarDeclaration *)): Likewise.
	(build_decl_tree): Likewise.