d: Fix ICE in gimplify_var_or_parm_decl, at gimplify.c:2755 (PR100882)

Constructor calls for temporaries were reusing the TARGET_EXPR_SLOT of a
TARGET_EXPR for an assignment, which later got passed to `build_assign',
which stripped away the outer TARGET_EXPR, leaving a reference to a lone
temporary with no declaration.

This stripping away of the TARGET_EXPR also discarded any cleanups that
may have been assigned to the expression as well.

So now the reuse of TARGET_EXPR_SLOT has been removed, and
`build_assign' now constructs assignments inside the TARGET_EXPR_INITIAL
slot.  This has also been extended to `return_expr', to deal with
possibility of a TARGET_EXPR being returned.

gcc/d/ChangeLog:

	PR d/100882
	* d-codegen.cc (build_assign): Construct initializations inside
	TARGET_EXPR_INITIAL.
	(compound_expr): Remove intermediate expressions that have no
	side-effects.
	(return_expr): Construct returns inside TARGET_EXPR_INITIAL.
	* expr.cc (ExprVisitor::visit (CallExp *)): Remove useless assignment
	to TARGET_EXPR_SLOT.

gcc/testsuite/ChangeLog:

	PR d/100882
	* gdc.dg/pr100882a.d: New test.
	* gdc.dg/pr100882b.d: New test.
	* gdc.dg/pr100882c.d: New test.
	* gdc.dg/torture/pr100882.d: New test.