[openmp] Fix warnings when building on Windows with latest MSVC or Clang ToT (#77853)

There were quite a few compilation warnings when building openmp on Windows with
the latest Visual Studios 2022 version 17.8.4. Some other warnings were visible
with the latest Clang at tip. This commit fixes all of them.

GitOrigin-RevId: 10f3296dd7d74c975f208a8569221dc8f96d1db1
8 files changed