llvm-ar: Fix MinGW compilation

llvm-ar is using CompareStringOrdinal which is available
only starting with Windows Vista (WINVER 0x600).

Fix this by hoising WindowsSupport.h, which sets _WIN32_WINNT
to 0x0601, up to llvm/include/llvm/Support and use it in llvm-ar.

Patch by Cristian Adam!

Differential revision: https://reviews.llvm.org/D74599