[OpenMP] Fix an OpenMP Windows build problem

When building OpenMP as part of LLVM, CMAKE was generating incorrect
location references for OpenMP build's first step's artifacts being used
in regenerating its Windows import library in the second step. The fix is
to feed a dummy non-buildable, rather than buildable, source to CMAKE to
satisfy its source requirements removing the need to reference the first
step's artifacts in the second step altogether.

Differential Revision:https://reviews.llvm.org/D146894