WindowsSupport.h: do not depend on private config header

WindowsSupport.h is a public header, however if it gets included, will cause a compile error indicating that llvm/Config/config.h cannot be found, because config.h is a private header. However there is no actual dependency on the private things in this header, so it can be changed to the public config header.

Reviewed By: amccarth

Differential Revision: https://reviews.llvm.org/D103370