Look through CXXBindTemporaryExprs when checking CXXFunctionCastExprs

for unused values.

This fixes a regression caused by r298676, where constructor calls to
classes with non-trivial dtor were marked as unused if the first
argument is an initializer list. This is inconsistent (as the test
shows) and also warns on a reasonbly common code pattern where people
just call constructors to create and immediately destroy an object.

llvm-svn: 298853