clang-cl: Invent a /winsysroot concept

On non-Windows platforms, --sysroot can be used to make the compiler use
a single, hermetic directory for all header and library files.

This is useful, but difficult to do on Windows. After D95472 it's
possible to achieve this with two flags:

    out/gn/bin/clang-cl win.c -fuse-ld=lld \
        /vctoolsdir path/to/VC/Tools/MSVC/14.26.28801 \
        /winsdkdir path/to/win_sdk

But that's still cumbersome: It requires two flags instead of one, and
it requires writing down the (changing) VC/Tools/MSVC version.

This adds a new `/winsysroot <dir>` flag that's effectively an alias to
these two flags. With this, building against a hermetic Windows
toolchain only needs:

    out/gn/bin/clang-cl win.c -fuse-ld=lld /winsysroot path

`/winsysroot <dir>` is the same as adding

    /vctoolsdir <dir>/VC/Tools/MSVC/<vctoolsver>
    /winsdkdir <dir>/Windows Kits/<winsdkmajorversion>

`<vctoolsver>` is taken from `/vctoolsversion` if passed, or else it's
the name of the directory in `<dir>/VC/Tools/MSVC` that's the highest
numeric tuple.

`<winsdkmajorversion>` is the major version in /winsdkversion if passed,
else it's the name of the directory in `<dir>/Windows Kits` that's the
highest number.

So `/winsysroot <path>` requires this subfolder structure:

  path/
    VC/
      Tools/
        MSVC/
          14.26.28801  (or another number)
            include/
            ...
    Windows Kits/
      10/
        Include/
          10.0.19041.0/ (or another number)
            um/
            ...
        Lib/
          10.0.19041.0/ (or another number)
            um/
              x64/
              ...
            ...

Differential Revision: https://reviews.llvm.org/D95534