[Offload] Check plugins aren't already deinitialized when tearing down (llvm#148642)

This is a hotfix for llvm#148615 - it fixes the issue for me locally.

I think a broader issue is that in the test environment we're calling
olShutDown from a global destructor in the test binaries. We should do
something more controlled, either calling olInit/olShutDown in every
test, or move those to a GTest global environment. I didn't do that
originally because it looked like it needed changes to LLVM's GTest
wrapper.

Author: callumfare

offload/liboffload/src/OffloadImpl.cpp

@@ -231,7 +231,7 @@ Error olShutDown_impl() {
 
   for (auto &P : OldContext->Platforms) {
     // Host plugin is nullptr and has no deinit
-    if (!P.Plugin)
+    if (!P.Plugin || !P.Plugin->is_initialized())
       continue;
 
     if (auto Res = P.Plugin->deinit())