This is so that the packaged toolchain can be used with clang-cl's
new /winsysroot flag added in https://reviews.llvm.org/rG82847436e.
No impact yet on the chrome build yet -- the tooling should
transparently get the new Windows Sdk path via the generated SetEnv
json file and use it with -imsvc. (I tested this locally by tweaking
my installed hermetic win package to look like the one generated
by this CL.)

Once this is deployed, we can switch the chrome build to use
/winsysroot if we want to -- it'd make compile command lines
a bit shorter and easier to work with.

Depends on https://chromium-review.googlesource.com/c/chromium/src/+/2665866

Bug: 1173176
Change-Id: I04c435f2323f26e3c26ed82656929809a7e0b5e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2655836Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>