<http://reviews.llvm.org/D22128> "Make CastExpr::getSubExprAsWritten look
through implicit temporary under CK_ConstructorConversion" was biting me again.
(I had originally developed loplugin:stringcopy against a Clang build that
includes my local fix for that issue.  I really need to see to get that resolved
upstream...)

(And while 95787416 "loplugin:stringcopy" was
actually a false positive, it doesn't hurt either, so just keep it.)

Change-Id: I726956adfbe67681005173cfdfed2e4b4cd6253d