[clang] [flang] [llvm] [CLANG][OpenMP] Add support for OpenMP6.0 transparent clause (PR #166810)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> The flang changes are actually not correct---what was there before was. The `transparent` clause takes an expression, not a constant.

Tht's is correct, it takes an "expression of OpenMP impex type" (not a keyword), which is an enum value.
<img width="689" height="414" alt="image" src="https://github.com/user-attachments/assets/0f1036b1-cdcc-4771-a883-543abff2c3e2" />
so the following is possible:
```cpp
void func(omp_impex_t arg) {
  #pragma omp task transparent(arg)
    ...
```

https://github.com/llvm/llvm-project/pull/166810