Revert r301089 "[builtins] Implement emulated TLS on Windows."

Revert r301089 "[builtins] Implement emulated TLS on Windows."

This broke the self-host build on Windows (PR32777).

Original commit message:

> [builtins] Implement emulated TLS on Windows.
>
> Summary:
> LLVM JIT needs to be able to use emulated TLS on all platforms, and this provides a reference one can compile to enable emutls for Linux/Mac/Windows.
>
> Reviewers: chh, howard.hinnant
>
> Reviewed By: chh
>
> Subscribers: mgorny, llvm-commits
>
> Differential Revision: https://reviews.llvm.org/D30787

llvm-svn: 301274