In this file is copy+pasted WindowsSupport.h from LLVM 12.0.1-rc1.
This is so that we can patch it. The upstream sources are incorrectly
including "llvm/Config/config.h" which is a private header and thus not
available in the include files distributed with LLVM.
The patch here changes it to include "llvm/Config/config.h" instead.
Patch submitted upstream: https://reviews.llvm.org/D103370