d: Return the correct value for C++ constructor calls (PR101664)

d: Return the correct value for C++ constructor calls (PR101664)

C++ constructors return void, even though the front-end semantic treats
them as implicitly returning `this'.  To handle this correctly, the
object reference is cached and used as the result of the expression.

	PR d/101664

gcc/d/ChangeLog:

	* expr.cc (ExprVisitor::visit (CallExp *)): Use object expression as
	result for C++ constructor calls.

gcc/testsuite/ChangeLog:

	* gdc.dg/extern-c++/extern-c++.exp: New.
	* gdc.dg/extern-c++/pr101664.d: New test.
	* gdc.dg/extern-c++/pr101664_1.cc: New test.