Fix build using setVirtualFileSystem (#577)

Update per llvm commit
llvm/llvm-project@30633f308941

Author: wenju-he

opencl_clang.cpp

@@ -235,7 +235,8 @@ Compile(const char *pszProgramSource, const char **pInputHeaders,
           new llvm::vfs::InMemoryFileSystem);
       OverlayFS->pushOverlay(MemFS);
 
-      compiler->createFileManager(std::move(OverlayFS));
+      compiler->setVirtualFileSystem(std::move(OverlayFS));
+      compiler->createFileManager();
       compiler->createSourceManager(compiler->getFileManager());
 
       // Create compiler invocation from user args before trickering with it