IntelLLVM: Add dependencies on system header files on Windows

In commit a90d2a9eed (IntelLLVM: Add support for Intel LLVM-based
compilers, 2020-11-02, v3.20.0-rc1~89^2~20) the IntelLLVM depfile
generation flags were taken from `Platform/Windows-Intel-C`.  Those
flags were added by commit a624a3e1b3 (Ninja: Use deps=gcc for Intel
Compiler on Windows, 2019-01-30, v3.14.0-rc1~30^2), which forgot to
account for commit 6d74e7870b (Ninja: Add dependencies on
system-provided header files, 2016-03-15, v3.6.0-rc1~265^2).

The `-QMD` option generates Makefile dependencies.  The `-QMMD` option
generates Makefile dependencies, but excludes system header files.
Part of the BuildDepends test includes a header, cmake_pch.hxx, that
includes a second header, zot_pch.hxx.  The test builds a pch file for
cmake_pch.hxx, touches zot_pch.hxx, then verifes that cmake_pch.hxx.pch
is regenerated based on the dependencies.

The cmake_pch.hxx contains `#pragma system_header` before it includes
zot_pch.hxx.  `#pragma system_header` indicates that the portion of the
file following the pragma is to be treated as a system header.

When `-QMMD` is used to generate dependencies, the `#include` of
zot_pch.hxx is ignored because it `-QMMD` says to ignore system headers.

Using `-QMD` instead uses all headers when generating dependencies
and causes this test to pass.  The Clang configuration in
Platform/Windows-Clang.cmake also uses the `-MD` option for generating
pre-compiled headers, instead of `-MMD`.

Signed-off-by: William R. Dieter <william.r.dieter@intel.com>

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