[cmake] Fix clang-cl cross-compilation on macOS

macOS paths usually start with /Users, which clang-cl interprets as a
macro undefine, leading to pretty much everything failing to compile.

CMake should be taught to put a -- in its compilation rules for clang-cl
(and I've been meaning to submit that upstream for a while). In the
meantime, however, and to support older CMake versions, we can just
create a custom make rules override to fix the compilation rules.

Differential Revision: https://reviews.llvm.org/D41219

llvm-svn: 320785

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