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When midl.py is finished its work it invokes midl.exe to ensure that the
results are the same. midl.exe defaults to invoking cl.exe, and we want
to consistently use clang-cl.exe.

This change uses /cpp_cmd to specify the clang-cl compiler. It then uses
/cpp_opt to specify -Wno-nonportable-include-path because some of the
SDK .idl files include files using different case from the file on disk.
Typical warnings look like this:

  include\10.0.19041.0\um\unknwn.idl(28,10): warning:
  non-portable path to file '"Unknwnbase.Idl"'; specified path differs
  in case from file name on disk [-Wnonportable-include-path]
  #include "unknwnbase.idl"
           ^~~~~~~~~~~~~~~~
           "Unknwnbase.Idl"

When you use /cpp_opt then you are also responsible for specifying -E
and -nologo and for passing along all of the /D commands, so this change
does that as well.

Bug: 1097510
Change-Id: Iece8ae7395d9afb05652d0a40f333721b3207a9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261267
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781638}