This check-in makes the following improvements to the OpenMP Windows build:

Only generate the second def file when necessary (native Windows import
library builds).
Properly clean up .def file artifacts.
Reduce the re-generated import library build artifacts to the minimum.
Refactor the import library related portions of the script for clarity.

Tested with MSVC and MinWG/gcc12.0

Differential Revision:https://reviews.llvm.org/D144419