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"clang -cc1" is the frontend only and as such it does not have any
include paths set. Use -Xclang <arg> instead.

[1] https://clang.llvm.org/docs/FAQ.html#id2

Change-Id: I372601cae5c672562b63fa07ed9e638e984fdd58
Reviewed-by: Marco Bubke <marco.bubke@qt.io>