[gn build] Use lld-link's new /winsysroot: flag

If `clang_base_path` is set, it must now point to a directory that contains
an lld-link built with D118070.

(If this is a problem for anyone, we can guard this behind a
lld_link_understands_winsysroot gn arg, but let's see if we can get away
without that for now.)

With this, it's possible to build everything in a normal cmd.exe Window,
an MSVC shell isn't needed \o/

(Assuming you set `clang_base_path`, and you set `sysroot` to a directory
that contains a win sysroot. If you have MSVC installed,
`python3 llvm\utils\sysroot.py make-fake --out-dir=my-sysroot` and
setting `sysroot = "//my-sysroot"` in args.gn works, for example.)

Differential Revision: https://reviews.llvm.org/D121871