Fix compilation of Any.h header.

In a previous patch I changed `std::decay<T>::type` to `std::decay<T>`
rather than `std::decay_t<T>`.  This seems to have broken the build
*only for clang-cl*.  I don't know why.

Submitting with post-commit review because this is an obvious fix for a
build breakage and we've verified that it fixes the breakage.