In !5594 (merged) the IntelLLVM depfile generation flags were taken from Platform/Windows-Intel-C. Those flags were added by !2893 (merged), which forgot to account for commit 6d74e787 (Ninja: Add dependencies on system-provided header files).
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.
Backport: release
Topic-rename: IntelLLVM-depfile-flags