Look through CXXBindTemporaryExprs when checking CXXFunctionCastExprs

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298853 91177308-0d34-0410-b5e6-96231b3b80d8