build: Add LLVM_WINSYSROOT to make setting /winsysroot easy on Win

Also add a script for sysroot management. For now, it can only create
fake sysroots that just symlink to local folders. This is useful for
testing.

Differential Revision: https://reviews.llvm.org/D96868

GitOrigin-RevId: 44ea794cf9ab8590a6abdf6d61622ba85e5ab1bc
3 files changed