For one, loplugin:unusedvariablecheck does not merely check for unused variables
with types from the standard library since
fe216494 "teach unusedvariablecheck plugin about
SfxPoolItem subclasses", so disabling loplugin:unusedvariablecheck based on
HAVE_GCC_ATTRIBUTE_WARN_UNUSED_STL is wrong.

For another, I have seen no standard library implementation that decorates its
types with such "warn-if-unused" attributes, and
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0600r0.pdf>
"[[nodiscard]] in the Library" (which proposes to add the corresponding C++17
attribute to just a few select functions and no types at all) makes it appear
unlikely that will happen.

Change-Id: I0a7759e1caf3e3137057c9689080948a4d6747e0