[llvm] Add default constructor of `llvm::ElementCount`.

This patch prevents failures like those reported in
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/34173.

We have enabled the default constructor for
`llvm::ElementCount` to make sure the code compiles on Windows.

Reviewed By: ormris

Differential Revision: https://reviews.llvm.org/D86240