[clang] [OpenMP] Support for `nothing` in `metadirective` (PR #73690)

[Sandeep Kosuri via cfe-commits]

================
@@ -12,7 +12,6 @@ int mixed() {
     x=d;
   }
 
-// expected-error at +2 {{#pragma omp nothing' cannot be an immediate substatement}}
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sandeepkosuri wrote:

oh sorry for missing that ! So, is this supposed to produce a compilation error (provided arch is x86_64)?

```
#pragma omp target
#pragma omp metadirective \
when( device={arch("x86_64")}: nothing) \
default(parallel)
for (int i= 0; i< N; i++) v3[i] = v1[i] * v2[i];
```

https://github.com/llvm/llvm-project/pull/73690