Fix truncated __OPENMP_NVPTX__ preprocessor condition

I don't have a test case for this but noticed this warning when including system headers with `-I` rather than `-isystem`.

```
In file included from <built-in>:1:
In file included from /nix/store/jq6bpm0xmhnbffhs5rkxq4n88g5xi2zg-clang-wrapper-11.0.1/resource-root/include/__clang_cuda_runtime_wrapper.h:157:
/nix/store/jq6bpm0xmhnbffhs5rkxq4n88g5xi2zg-clang-wrapper-11.0.1/resource-root/include/__clang_cuda_math.h:39:25: warning: extra tokens at end of #ifdef directive [-Wextra-tokens]
                        ^
```

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D95299

Author: r-burns

clang/lib/Headers/__clang_cuda_math.h

@@ -36,7 +36,7 @@
 // because the OpenMP overlay requires constexpr functions here but prior to
 // c++14 void return functions could not be constexpr.
 #pragma push_macro("__DEVICE_VOID__")
-#ifdef __OPENMP_NVPTX__ && defined(__cplusplus) && __cplusplus < 201402L
+#if defined(__OPENMP_NVPTX__) && defined(__cplusplus) && __cplusplus < 201402L
 #define __DEVICE_VOID__ static __attribute__((always_inline, nothrow))
 #else
 #define __DEVICE_VOID__ __DEVICE__