bugfix: Fix axom's MSVC with OpenMP build

[johnbowen42]

Axom found some bugs in RAJA's latest release with two root causes:
#1884
#1947

The first led to MSVC attempting to compile some OpenMP pragmas it didn't understand. The second led to undefined symbol names. Both of these are ultimately strange, because our tests and examples do not even compile with ENABLE_OpenMP=On with msvc. Axom's Windows workflow is to build RAJA without any of the tests and exercises enabled, install RAJA, then only use non-reduction versions of RAJA's OpenMP constructs. This only ever worked because SFINAE and code duplication allowed reduction-free OpenMP code to be compiled by Axom downstream, but RAJA's reduction versions of launch, forall and kernel to go uncompiled.

tldr; RAJA + OpenMP + msvc doesn't work in most cases, but axom carved out some use cases that do. This PR retains that functionality.

We might need to add more #ifdef guards to the launch and forall backends

@bmhan12

[johnbowen42]

this pipeline #1975 should catch problems like the one from #1947 in the future, but the error the cpp 17 PR introduced won't be tested by this. Our only option for testing that would be to add a Windows-only test, because many tests and examples do not compile with Windows and OpenMP

[bmhan12]

Can confirm these changes do fix the failures Axom was seeing in llnl/axom#1769 !